Use localized strings

Hi Drew,

this pull request moves all hard-coded strings to `nls/` so they can be localized.
I didn‘t bother about the strings in dialog.html which are commented out, since
they aren‘t in use yet. Let me know if you‘d liked them to be localized as well.

Rebased commits:
- Move all hard-coded UI strings to nls/root
- Move hard-coded UI strings from dialog to nls/