I guess you're missing some of language identifiers that mediawiki supports. It's my guess the full list is here:
https://github.com/Wikia/app/blob/dev/languages/Names.php

One of them is "be-tarask" which has the corresponding language class here:
https://github.com/Wikia/app/blob/dev/languages/classes/LanguageBe_tarask.php

which in turn has a definition on convertPlural method, but is missing here while you still included "be".


I think this list of languages needs another pass focused on checking its completeness.