fix another undefined variable

app.py

@@ -73,7 +73,10 @@ if __name__ == "__main__":
 
   st.write(f"It supports converting many writing sytems to IPA symbols, including approximately {len(valid_epitran_mappings)} languages/scripts, listed below:")
   index_of_desired_default = valid_epitran_mappings.index("swa-Latn")
-  selected_mapping = st.selectbox("Select input language/script:", valid_epitran_mappings, index=index_of_swa_latn, format_func=get_lang_description_from_mapping_name)
+  selected_mapping = st.selectbox("Select input language/script:", 
+                                  valid_epitran_mappings, 
+                                  index=index_of_desired_default, 
+                                  format_func=get_lang_description_from_mapping_name)
 
   
   description = get_lang_description_from_mapping_name(selected_mapping)