Improvements to Language/ Node::language property

[oxisto]

There are some issues that we have with language that we should tackle at some point

• Node::language is a nullable field, which is kind of annoying and we had issues in the past where we didn't catch that it was null and things went horribly wrong. Instead I would propose an UnknownLanguage similar to the UnknownType (or ProblemLanguage?)
• We currently do not set the language on component and translation result, which is understandable because it could potentially be multiple languages; but in an analysis the information that this for example is a pure python component (or which languages) are in there is definitely interesting. It might also be interesting for certain passes. I would therefore propose to a) set the language if all sub-nodes are of one language b) introduce something like a MultipleLanguages language which contains all languages used