Reported in https://translatewiki.net/wiki/Thread:Support/Saraiki_language_%3F. This message should not happen, as we pre-cache the results. But if one requests data for language with no activity, it will perform a slow database query, as no data has been cached.
The script should be amended so that it does negative caching for a list of languages for which there were no activity. It's hard to get list for "isKnownLanguageTag", so maybe use TranslateUtils::getLanguages instead. The special page should validate input against the same list.