Keyword searches with Low Search are diacritic insensitive. That is to say, both the collection index and given search terms go through an algorithm to strip them out or replace them with unaccented equivalents. LS uses EE's native $foreign_characters array for this, located in /system/expressionengine/config/foreign_chars.php.
For example, take the word ...
The point is that EECMS uses two folders of languages!
One is /system/expressionengine/language/ and the other is /system/codeigniter/system/language. Both language packs are completely different.
The dates and the form validation messages are used from the one in the Codeigniter folder.
You can find CodeIgniter Language Packs on github, but I don't know ...
I've had a look around and could not find language packs for either language, sorry (There is an Italian version but the price is high!).
I'm not aware of the 'minimum' list for front end translations either (lots on negative answer here I'm afraid). I do hope someone can give you a link to a positive answer though.
It might be down to you doing it ...
There's always the Custom System Messages addon - http://devot-ee.com/add-ons/custom-system-messages - which allows you to run things like error messages through regular templates. That way EE upgrades don't affect the message text.