Cache should respect language selection

[adamcik]

When the language selector is used, the cached page (in the wrong language) shows up. Instead, language should be changed right away.

Possible solutions:

• adding languagecode to cache realm
• invalidating cache on language selection

[adamcik]

CacheMiddleware now creates a cache wrapper that adds both realm prefix and language to the key automaticly. (72b3a8ba)