Change language manually

[WofWca]

Currently it only depends on the browser's language.

Need this because a user may consider a translation to be of poor quality. Or because there is no translation in the user's language, but there is one in another non-english language that the user understands (e.g. Chinese Simplified or Traditional). Or if for some reason it doesn't display the extension in the browser's language (e.g. I messed up language codes).

There may be problems implementing this, because of string placeholders.