Add language catalog selector

docs/PRODUCT.md

@@ -41,6 +41,8 @@ and writing engine, then prepares final lyrics and style instructions.
 -   User controls vocal gender/type, rap/singing mode, delivery and
     section-specific performance.
 -   Multiple languages and dialects can be used in one song.
+-   Languages can be selected from a large local catalog with search,
+    category browsing and checkbox multi-select.
 -   Arabic receives dedicated diacritics/harakat/tanween handling for
     pronunciation.
 -   Energy and emotional progression can change throughout the song.
@@ -94,6 +96,13 @@ presets. Users can search, browse by category and select or deselect
 multiple vocalists with checkboxes. Each preset supplies a role label,
 voice type, gender selection and performance style for the Song Project.
 
+## Language Selector
+
+The application provides a large curated local language catalog. Users
+can search, browse by regional category and select or deselect multiple
+languages with checkboxes. Removing a language also removes any dialect
+entries attached to it from the Song Project.
+
 ## Project Management
 
 Every song is stored as a project. Important changes can create versions