I use Polish (my and my students mother tongue) as I teach beginners... And I think to some point there is a need to use mother tongue to explain grammar. Of course all the instructions are in English and they have to learn them. IMHO the FCE level is agood point to use only English in class - that how I was taught. From the FCE level (B2 of Council of Europe level) I had classes with native speaker so there was no choice
And at that level I was ready to be taught only in English.