Compile localizations with build.rs

[lhecker]

This will allow us to add more languages.

[lhecker]

@DHowett I'd be particularly curious about your opinion on

• where to put the original translation strings
• whether the current file structure makes sense (i.e. grouped by key instead of language)
• and perhaps whether to use TOML or YAML (both are fine with me)

[DHowett]

go go go