For Romanians and Moldavian I don't see: ă â ș ț î

This is an impossible task, just use the standard "Programmers" layout for your language which maps to US and allows access to accented characters via modifiers. The entire notion of a "local" keyboard layout sounds like a nationalistic throwback to the days where the world was much smaller. Some languages really need it, but most using the Latin alphabet simply don't, just stick to US.

I think you're supposed to use the composition key for those. Don't know how frequent they are in croatian though, so it might be impairing the speed at which you can type if they are very common

You can get these with the composition keys. Shift+AltGr = ˇ and then hit the letter to compose them