is 'strong' a good translation of 'mạnh mẽ'? I usually hear this used to mean 'strong willed' or 'headstrong' - that kind of thing. I've never heard it used for physical strength.
It can mean physical strength, yes.
I read manga in vietnamese, and there the translator just uses "mạnh" (usually superheroes thought)
The drop-down hint says 'powerful' for 'mạnh mẽ' but it's not accepted.