About [[MediaWiki:Content-failed-to-parse/en]]
It is currently impossible to translate it with proper inflection due to how parameters are inserted. Of course, this can be avoided with some clever structures and a lot of colons, as I've attempted to do, but it sounds weird nevertheless (I'm translating to Polish).
And also MediaWiki:Bad-target-model/pl.
Translation is probably possible if you phrase it like this:
Failed to parse content of type $2 for model with MIME type $2: $3