try this

app.py

@@ -53,7 +53,7 @@ with gr.Blocks() as demo:
     with gr.Row():
         search_input = gr.Textbox(label="Search & Selected Languages", placeholder="Type to search and add languages...")
         dropdown = gr.Dropdown(label="Available Languages", choices=languages.get_all_supported_languages(), interactive=True, allow_custom_value=False)
-        search_input.change(filter_languages, [search_input, search_input], dropdown)
+        search_input.change(filter_languages, [search_input, search_input], [dropdown, dropdown])
         dropdown.select(add_language, [dropdown, search_input], search_input)
     with gr.Row():
         is_reverse = gr.Checkbox(label="Reverse Phonemization")