diff --git a/app/Controllers/LocaleSwitcher.php b/app/Controllers/LocaleSwitcher.php
index 490a211..25a35ff 100644
--- a/app/Controllers/LocaleSwitcher.php
+++ b/app/Controllers/LocaleSwitcher.php
@@ -11,10 +11,13 @@ use App\Core\Locale;
  * Renders the language switcher widget. Pure HTML generation — no
  * side effects, no header writing.
  *
- * Output is semantic HTML (a <ul> of links) with `aria-current` for the
- * active locale, `hreflang` for SEO, and `lang` for screen readers.
- * The basic list-of-locale-codes is the MVP. Sub-Issue D (responsive
- * SVG flag UI) refines the presentation.
+ * Each option gets:
+ *   - an inline 24×16 SVG flag (sub-Issue D),
+ *   - `hreflang` and `lang` for SEO and screen readers,
+ *   - `aria-current="true"` on the active option.
+ *
+ * Active option is rendered as a <span> (not a link) so it cannot
+ * be reactivated. All options are ≥44px touch targets via CSS.
  */
 final class LocaleSwitcher
 {
@@ -26,7 +29,7 @@ final class LocaleSwitcher
 
     public function render(): string
     {
-        $path = $this->sanitisePath($this->currentPath);
+        $path      = $this->sanitisePath($this->currentPath);
         $ariaLabel = htmlspecialchars(
             I18n::t('locale.switcher.aria', [], $this->currentLocale),
             ENT_QUOTES,
@@ -36,17 +39,35 @@ final class LocaleSwitcher
         $html  = '<ul class="locale-switcher" role="list" aria-label="' . $ariaLabel . '">';
         foreach (Locale::SUPPORTED as $code) {
             $isCurrent = $code === $this->currentLocale;
-            $name      = htmlspecialchars(I18n::t('locale.' . $code, [], $this->currentLocale), ENT_QUOTES, 'UTF-8');
+            $name      = htmlspecialchars(
+                I18n::t('locale.' . $code, [], $this->currentLocale),
+                ENT_QUOTES,
+                'UTF-8',
+            );
             $codeAttr  = htmlspecialchars($code, ENT_QUOTES, 'UTF-8');
+            $flag      = self::flagSvg($code);
 
-            $html .= '<li' . ($isCurrent ? ' class="is-current" aria-current="true"' : '') . '>';
+            $classes = 'locale-switcher__option';
             if ($isCurrent) {
-                $html .= '<span lang="' . $codeAttr . '">' . $codeAttr . '<span class="visually-hidden"> (' . $name . ')</span></span>';
+                $classes .= ' is-current';
+            }
+
+            $html .= '<li class="locale-switcher__item">';
+            if ($isCurrent) {
+                $html .= '<span class="' . $classes . '" aria-current="true" lang="' . $codeAttr . '">'
+                       . $flag
+                       . '<span class="locale-switcher__label">' . $name . '</span>'
+                       . '</span>';
             } else {
                 $url = '/locale?set=' . rawurlencode($code) . '&amp;return=' . rawurlencode($path);
-                $html .= '<a href="' . $url . '" hreflang="' . $codeAttr . '" lang="' . $codeAttr . '" rel="alternate">'
-                       . $codeAttr
-                       . '<span class="visually-hidden"> (' . $name . ')</span>'
+                $html .= '<a class="' . $classes . '"'
+                       . ' href="' . $url . '"'
+                       . ' hreflang="' . $codeAttr . '"'
+                       . ' lang="' . $codeAttr . '"'
+                       . ' rel="alternate"'
+                       . '>'
+                       . $flag
+                       . '<span class="locale-switcher__label">' . $name . '</span>'
                        . '</a>';
             }
             $html .= '</li>';
@@ -56,6 +77,49 @@ final class LocaleSwitcher
         return $html;
     }
 
+    /**
+     * Inline 24×16 SVG for the four supported locales.
+     *
+     * - DE: black/red/gold horizontal stripes (Germany)
+     * - EN: simplified Union Jack (en-GB per ADR-002)
+     * - UK: blue/yellow horizontal stripes (Ukraine)
+     * - RU: white/blue/red horizontal stripes (Russia)
+     *
+     * Decorative: marked `aria-hidden="true"` + `focusable="false"`.
+     * The full accessible name is conveyed by the visible label and
+     * the <a>'s hreflang/lang.
+     */
+    public static function flagSvg(string $locale): string
+    {
+        $svg = match ($locale) {
+            'de' => '<svg class="flag" viewBox="0 0 24 16" aria-hidden="true" focusable="false">'
+                  . '<rect width="24" height="5.33" fill="#000"/>'
+                  . '<rect y="5.33" width="24" height="5.34" fill="#DD0000"/>'
+                  . '<rect y="10.67" width="24" height="5.33" fill="#FFCC00"/>'
+                  . '</svg>',
+            'en' => '<svg class="flag" viewBox="0 0 24 16" aria-hidden="true" focusable="false">'
+                  . '<rect width="24" height="16" fill="#012169"/>'
+                  . '<path d="M0,0 L24,16 M24,0 L0,16" stroke="#fff" stroke-width="2.4"/>'
+                  . '<path d="M0,0 L24,16 M24,0 L0,16" stroke="#C8102E" stroke-width="1.2"/>'
+                  . '<path d="M12,0 V16 M0,8 H24" stroke="#fff" stroke-width="3.2"/>'
+                  . '<path d="M12,0 V16 M0,8 H24" stroke="#C8102E" stroke-width="1.6"/>'
+                  . '</svg>',
+            'uk' => '<svg class="flag" viewBox="0 0 24 16" aria-hidden="true" focusable="false">'
+                  . '<rect width="24" height="8" fill="#005BBB"/>'
+                  . '<rect y="8" width="24" height="8" fill="#FFD500"/>'
+                  . '</svg>',
+            'ru' => '<svg class="flag" viewBox="0 0 24 16" aria-hidden="true" focusable="false">'
+                  . '<rect width="24" height="5.33" fill="#fff"/>'
+                  . '<rect y="5.33" width="24" height="5.34" fill="#0039A6"/>'
+                  . '<rect y="10.67" width="24" height="5.33" fill="#D52B1E"/>'
+                  . '</svg>',
+            default => '<svg class="flag" viewBox="0 0 24 16" aria-hidden="true" focusable="false">'
+                     . '<rect width="24" height="16" fill="#888"/>'
+                     . '</svg>',
+        };
+        return $svg;
+    }
+
     /**
      * Make sure the path is safe to embed as a query string value and
      * a redirect target. Drops query/fragment, keeps only the path.
diff --git a/app/Core/I18n.php b/app/Core/I18n.php
index 637ee13..7a151f2 100644
--- a/app/Core/I18n.php
+++ b/app/Core/I18n.php
@@ -48,14 +48,6 @@ final class I18n
         }
         $text ??= $key;
         return self::interpolate($text, $params);
-
-        if ($params !== []) {
-            $search  = array_map(static fn (string $k): string => '{' . $k . '}', array_keys($params));
-            $replace = array_values($params);
-            $text    = str_replace($search, $replace, $text);
-        }
-
-        return $text;
     }
 
     /**
diff --git a/public/css/haus-schleusingen.css b/public/css/haus-schleusingen.css
index 54a4aa7..1e4252a 100755
--- a/public/css/haus-schleusingen.css
+++ b/public/css/haus-schleusingen.css
@@ -1316,3 +1316,122 @@ footer {
     text-align: center;
   }
 }
+
+/* LOCALE SWITCHER (sub-Issue D) */
+.locale-switcher {
+  display: flex;
+  align-items: center;
+  gap: 0.25rem;
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.locale-switcher__item {
+  display: flex;
+}
+
+.locale-switcher__option {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.4rem;
+  min-height: 44px;
+  min-width: 44px;
+  padding: 0.45rem 0.55rem;
+  border: 1px solid transparent;
+  border-radius: 4px;
+  text-decoration: none;
+  font-size: 0.78rem;
+  font-weight: 500;
+  letter-spacing: 0.04em;
+  color: rgb(255 255 255 / 90%);
+  background: transparent;
+  transition:
+    background 0.2s,
+    border-color 0.2s,
+    color 0.2s,
+    transform 0.15s;
+  cursor: pointer;
+}
+
+.locale-switcher__option:hover,
+.locale-switcher__option:focus-visible {
+  background: rgb(255 255 255 / 14%);
+  border-color: rgb(255 255 255 / 30%);
+  color: var(--white);
+  transform: translateY(-1px);
+}
+
+.locale-switcher__option.is-current {
+  background: rgb(255 255 255 / 20%);
+  border-color: var(--accent-light);
+  color: var(--white);
+  cursor: default;
+}
+
+.locale-switcher__option.is-current:hover {
+  transform: none;
+}
+
+nav.scrolled .locale-switcher__option {
+  color: var(--stone);
+}
+
+nav.scrolled .locale-switcher__option:hover,
+nav.scrolled .locale-switcher__option:focus-visible {
+  background: var(--warm);
+  border-color: var(--stone);
+  color: var(--dark);
+}
+
+nav.scrolled .locale-switcher__option.is-current {
+  background: var(--cream);
+  border-color: var(--accent);
+  color: var(--dark);
+}
+
+.locale-switcher__option .flag {
+  width: 24px;
+  height: 16px;
+  flex: 0 0 24px;
+  border-radius: 2px;
+  box-shadow: 0 0 0 1px rgb(0 0 0 / 15%);
+  display: block;
+}
+
+.locale-switcher__label {
+  white-space: nowrap;
+}
+
+/* Hide labels on small screens, keep the 44px flag target */
+@media (width <= 720px) {
+  .locale-switcher__label {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border: 0;
+  }
+
+  .locale-switcher__option {
+    min-width: 44px;
+    padding: 0.55rem;
+  }
+}
+
+/* VISUALLY HIDDEN (a11y) */
+.visually-hidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border: 0;
+}
diff --git a/tests/Controllers/LocaleSwitcherTest.php b/tests/Controllers/LocaleSwitcherTest.php
new file mode 100644
index 0000000..2db00e8
--- /dev/null
+++ b/tests/Controllers/LocaleSwitcherTest.php
@@ -0,0 +1,128 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Controllers;
+
+use App\Controllers\LocaleSwitcher;
+use App\Core\Locale;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Renders the language switcher widget and checks that:
+ *   - 4 items, one per supported locale,
+ *   - the active locale is marked aria-current="true" and is a <span>,
+ *   - inactive locales are <a> links to /locale?set=...&return=...,
+ *   - every item contains a flag SVG,
+ *   - the rendered label is in the current locale's language.
+ */
+final class LocaleSwitcherTest extends TestCase
+{
+    #[Test]
+    public function rendersFourItemsForAllSupportedLocales(): void
+    {
+        $html = (new LocaleSwitcher('en', '/'))->render();
+        self::assertStringContainsString('<ul class="locale-switcher"', $html);
+
+        // The 3 inactive locales render as <a hreflang="..">. The active
+        // locale renders as <span lang=".."> (no hreflang). Together all
+        // 4 must be present in either form.
+        foreach (Locale::SUPPORTED as $code) {
+            self::assertTrue(
+                str_contains($html, 'hreflang="' . $code . '"') || str_contains($html, 'lang="' . $code . '"'),
+                "locale '$code' is missing from switcher",
+            );
+        }
+        self::assertSame(4, substr_count($html, 'class="flag"'), 'expected 4 flag SVGs');
+    }
+
+    #[Test]
+    public function marksCurrentLocaleWithAriaCurrentAndSpan(): void
+    {
+        $html = (new LocaleSwitcher('uk', '/'))->render();
+        self::assertStringContainsString('is-current', $html);
+        self::assertStringContainsString('aria-current="true"', $html);
+        self::assertStringContainsString('lang="uk"', $html);
+
+        // active option must be a <span>, not an <a>
+        self::assertMatchesRegularExpression(
+            '/<span class="locale-switcher__option is-current"[^>]*aria-current="true"[^>]*lang="uk"/',
+            $html,
+        );
+    }
+
+    #[Test]
+    public function inactiveLocalesAreLinksToLocaleController(): void
+    {
+        $html = (new LocaleSwitcher('de', '/foo/bar'))->render();
+        self::assertStringContainsString('href="/locale?set=en&amp;return=%2Ffoo%2Fbar"', $html);
+        self::assertStringContainsString('href="/locale?set=uk&amp;return=%2Ffoo%2Fbar"', $html);
+        self::assertStringContainsString('href="/locale?set=ru&amp;return=%2Ffoo%2Fbar"', $html);
+    }
+
+    #[Test]
+    public function stripsQueryAndFragmentFromReturnPath(): void
+    {
+        $html = (new LocaleSwitcher('de', '/?lang=uk#kontakt'))->render();
+        // sanitisePath keeps only the path part
+        self::assertStringContainsString('return=%2F', $html);
+        self::assertStringNotContainsString('return=%2F%3Flang', $html);
+        self::assertStringNotContainsString('return=%2F%23kontakt', $html);
+    }
+
+    #[Test]
+    public function rejectsPathsThatDoNotStartWithSlash(): void
+    {
+        $html = (new LocaleSwitcher('de', 'https://evil.example/'))->render();
+        // sanitisePath falls back to '/'
+        self::assertStringContainsString('return=%2F', $html);
+        self::assertStringNotContainsString('evil.example', $html);
+    }
+
+    /**
+     * @return array<string, array{string, string}>
+     */
+    public static function flagDataProvider(): array
+    {
+        return [
+            'DE Germany'      => ['de', '#FFCC00'],
+            'EN UnionJack'    => ['en', '#C8102E'],
+            'UK Ukraine'      => ['uk', '#FFD500'],
+            'RU Russia'       => ['ru', '#D52B1E'],
+        ];
+    }
+
+    #[Test]
+    public function flagSvgReturnsValidSvgForEverySupportedLocale(): void
+    {
+        foreach (Locale::SUPPORTED as $code) {
+            $svg = LocaleSwitcher::flagSvg($code);
+            self::assertStringStartsWith('<svg', $svg);
+            self::assertStringContainsString('viewBox="0 0 24 16"', $svg);
+            self::assertStringContainsString('aria-hidden="true"', $svg);
+            self::assertStringContainsString('focusable="false"', $svg);
+            self::assertStringContainsString('class="flag"', $svg);
+            self::assertStringEndsWith('</svg>', $svg);
+        }
+    }
+
+    #[Test]
+    public function flagSvgHasFallbackForUnknownLocale(): void
+    {
+        $svg = LocaleSwitcher::flagSvg('xx');
+        self::assertStringStartsWith('<svg', $svg);
+        self::assertStringContainsString('class="flag"', $svg);
+        self::assertStringEndsWith('</svg>', $svg);
+    }
+
+    #[Test]
+    public function ariaLabelUsesCurrentLocaleName(): void
+    {
+        $htmlDe = (new LocaleSwitcher('de', '/'))->render();
+        $htmlEn = (new LocaleSwitcher('en', '/'))->render();
+
+        self::assertStringContainsString('aria-label="Sprache wählen"', $htmlDe);
+        self::assertStringContainsString('aria-label="Choose language"', $htmlEn);
+    }
+}