Localization improvements 2

[aatkin]

Continuation to #3241

• [ ] Document current translation parameters

Markdown documentation created automatically, similar to application permissions? Or show preview of current translations in a separate page, with default values filled in?

• [ ] Validate translation parameters

Previous implementation (which is now removed) checked only that if :en translation had parameters "%1 %2 %3", that same key for other locales had the exact same parameters.