Fix problem viewer for Norwegian (and some other languages)

[stianv]

The JavaScript problem viewer will not show up on browsers with e.g. Norwegian as language preference, since /js/tr/nb.js doesn't exist. Suspect this is also the case for other languages where no internationalization is currently provided. Could this site fall back to /js/tr/en.js for these languages? Workaround until that time would be for users to alter the language preferences in the browser like this https://www.w3.org/International/questions/qa-lang-priorities. I think this is quite urgent, given that people with these language preferences would be likely to give up on the site (which would be a shame since the site is so good )