As others have said, you must always follow the rules of your target language when regionalizing.
The only possible reason (other than not realising that title case is an English-only idiom) that I can think of for wanting idiomatic consistency between languages is that an inconsistency is visible anywhere that you have incomplete regionalization: where only some of the text and graphics have been changed to local forms, while others remain in the "source language" - typically English.
This is hard to avoid unless you can consistently get every single resource translated to every single target language before every single update or release - which means, on regularly updated websites in particular, this is a very common problem indeed.
So you get adjacent items in different languages and idioms, like:
- Helderheid en actergrond
- Beer bier
- Wet Water
This looks weird, obviously. But unfortunately, there's no good way around it that I am aware of.
There are scripts that can title-case, and you could choose to only make the script run when the user is viewing in English... but such scripts can never be perfect, and anyway won't fix things like graphics, multimedia content, etc.
And even if you did, it would then be unclear which is translated. At least with "Beer bier", people have a chance to understand "this is probably Dutch, it means 'A beer named after bears' not the English meaning of 'a corpse-plinth made out of beer cans'."
So there really is no easy solution for incomplete translation scenarios.