Language overhaul

[pietroalbini]

Currently botogram support internationalization for its default messages, but that feature is available for a few languages only (Italian and English), it's undocumented, and it's really limited (no way to override messages without modifying the .po files, rebuilding the package and reinstalling it).

This issue tracks the progress to improve this situation.

• Add narrative documentation about languages
• Add some documentation about adding support for new languages
• Add a way to override language values without recompiling the .po files (issue Add a way to override language strings without recompiling .po files #47)