Just ask them to keep a lower profile. Tell them that you are still a beginner and you will not manage to understand their language at this level.
Usually they should react by using simpler English,but some don't really care and go on as if you said nothing. That is many times due to them being inconsiderate or on the other hand they are not professional enough to adjust their level to suit their partner.
In this case if they are not reactive to your warning , consider looking for another suitable partner ,being suitable after all is partly being able to suit your needs ,when learning a language.