In Greece, you do not have to worry about this.
On all of the gulets which our agency offers in Greece 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) also they 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. In Turkey, this can sometimes be a problem but for Greece, you do not have anything to worry about.