Implement language translation

[isedwards]

Decide whether to implement language translation solely server side using the standard Django tools, or whether to (also) implement a javascript solution that allows real-time language changes without page reload. The later would be useful in the case where someone changes language whilst they are modifying the page contents, it could be implemented with a simple ajax request.

We will us the translation files that were originally created for Climsoft Desktop: climsoft/Climsoft#592