Dia means both he and she. In finnish is the same thing "hän" means both he and she so this is easy for me :)
How can one differentiate between he and she in Indonesian?
there's no he & she in Indonesian, just "dia" (neutral gender)
Does kuat mean strength of all kinds? As in emotional strength / perseverence as well as physical? Or are there other words for those things separately?
Yeah, include in emotional as well as physical. So it is actually need more words to differentiate it (more like to emphasize it).
Emotional strength = Kekuatan secara emosional
Physical strength = Kekuatan secara fisik