In Croatia, you do not have to worry about this. On all of the gulets which our agency offers in Croatia at least 1 person (usually more or all of the crew) speaks very good (Active) English.
Maybe the grammar will not always be perfect, but the communication will not be a problem.
On several gulets (especially more luxury ones) they also speak French, Italian, German, Russian or Spanish.
However, for this, you always have to double check in advance.
If you request a gulet where they can speak your language (which is not English) let us know and we will try to find it.
We can also put a guide who speaks your language on board with you for 7 days and he will help you in communication with the crew.
In Turkey, this can sometimes be a problem but for Croatia, you do not have anything to worry about it.