Replace deprecated Language::fetchLanguageNames/fetchLanguageName/isSupportedLanguage

[umherirrender]

Issue

This extension is using the deprecated functions Language::fetchLanguageNames, Language::fetchLanguageName and Language::isSupportedLanguage
Please replace with the LanguageNameUtils service, available since 1.34

The function is going to be hard deprecated and would emit deprecation warning, which can result in massive log spam.

https://phabricator.wikimedia.org/T325962
https://phabricator.wikimedia.org/T325974