How should Themes handle Internationalization?

Currently, Internationalization is recommended, and if internationalization is incorporated, Themes are required to use theme-slug as a unique textdomain.

What else should be considered? For example:

1. If Themes include translation, should all strings be required to be translatable?
2. If Themes do not include translation, should any strings be translatable?
3. What else should be considered?

In other words: what constitutes best-practice, and what should be avoided, when implementing Theme internationalization?

#discussion-topic, #guidelines, #internationalization