When I travelled Japan I always started off talking to Japanese people with the following sentence:
This would always get the typical "oh" and nodding heads in answer and then they would start to blabber in their normal speed and difficulty. Of course absolutely incomprehensible to me as a Japanese language beginner.
As it seems a typical construct (maybe as a form of politeness) to have little excusing introductions in Japanese. I thought it might be so custom to a Japanese person. That they just "overhear" what I'm actually saying and just listen to the fact that I'm talking Japanese. Maybe similar to "sorry for interrupting...".
Anyway, what do you think is a good way to actually make clear to a Japanese person. That you really can't speak Japanese well and they have to talk slowly and in simple sentences?