We have a multilingual website based on Sitecore 8.1 rev. 151207 (8.1 Update-1) and Glass Mapper. It has four languages: English, Danish, Swedish and German.
Some of the developers are native English speakers, and some are native German speakers. "Naturally", they used their respective Sitecore UI languages in the content editor when creating and editing templates. So we ended up with approximately 100 item templates, with some of them having only an English version, some only German, and some both. The same goes for template fields—actually, some templates have most fields with two language versions, a couple fields in only English or German.
This is a messy situation, and I believe that mid-term we should spend time on making all templates English-only. But in the meanwhile, I would like to know whether or not this could become an issue short-term. Could this result in errors in some situations? Could it affect language fallback functionality? How much should we prioritize fixing this?