raise errors when the source or target language is not chosen

app.py

@@ -38,7 +38,11 @@ def load_tokenizer(src_lang, tgt_lang):
 # cache function
 @lru_cache(maxsize=100)
 def translate(text: str, src_lang: str, tgt_lang: str):
-    return _translate(text, src_lang,tgt_lang )
+    if not src_lang:
+        raise gr.Error("The source language is empty! Please choose it in the dropdown list.")
+    if not tgt_lang:
+        raise gr.Error("The target language is empty! Please choose it in the dropdown list.")
+    return _translate(text, src_lang, tgt_lang)
 
 # Only assign GPU if cache not used
 @spaces.GPU