Generally we say yes for one reason. We want to help each other, communicate together and treat the patients as good as we can. Therefore it is useful to speak the local language.
Anyways there are some countries you may speak not the local language, but the majority of the local residents speaks an international language like English, France or Spanish.
If you don’t speak the local language of a country but want to visit it so badly, try your best and ask a host.
There’s no harm in asking!
In most cases a host will list all the languages they speak.