For a non-english single-language site, is it still best practice to write t()-strings in the theme in English?
If you're 100% sure that you do not need to (re-)use this code in a multi-language environment, now or in the future, you're safe using foreign language strings. Personally, I consider it best practice to always use t() because you never know what's going to happen in the future.

Be aware that strings in code that are run through t(), are assumed to be in English, so you have to choose between t('English text') or 'Svensk text'. Never use t('Svensk text').