Improve language display in LanguageSelector

Replaced the placeholder arrow in the language selector button with the current language name using the new `GetLanguageName` method. Added `GetLanguageName` to map culture codes to language names. Fixed encoding issue by correcting "Fran�ais" to "Français" in the French language option.

EnvelopeGenerator.ReceiverUI/Shared/LanguageSelector.razor

@@ -7,7 +7,7 @@
 <div class="language-selector">
     <button class="language-selector__trigger" @onclick="ToggleDropdown" aria-label="Select Language">
         <span class="fi fi-@GetFlagCode(CurrentCulture)"></span>
-        <span class="language-selector__arrow">?</span>
+        <span class="language-selector__arrow">@GetLanguageName(CurrentCulture)</span>
     </button>
     
     @if (isOpen)
@@ -23,7 +23,7 @@
             </button>
             <button class="language-selector__option" @onclick="@(() => ChangeLanguageAsync("fr-FR"))">
                 <span class="fi fi-fr"></span>
-                <span>Français</span>
+                <span>Français</span>
             </button>
         </div>
     }
@@ -59,4 +59,15 @@
             _ => "de"
         };
     }
+
+    private string GetLanguageName(string culture)
+    {
+        return culture switch
+        {
+            "de-DE" => "Deutsch",
+            "en-US" => "English",
+            "fr-FR" => "Français",
+            _ => "Deutsch"
+        };
+    }
 }