Remove too early metadata language definition without try-except block

[HoLuLuLu]

Fixes: #289

Changes explaination:

• definition of self.metadata.language throws MnamerException if path_data.get("language") is not a valid language (in my case it was the German word "und", which means "and" and is not a language reference at all)
• later if condition isn't necessary because self.metadata.language will never be set before
• try-except block used makes sure that path_data.get("language") can be converted to a Language, otherwise the metadata language will not be set