Usually I end up saying
我并非/不是以中文为母语的人,(...so please forgive my mistakes).
Is there any way to avoid the awkward-seeming "以中文为母语" construction here but still get this point across, without saying something totally different like "I'm a foreigner" - there are other situations where I really do need to be referring to the specific level of Chinese language capability. It's obvious that I'm not, but I still feel the need to say something along these lines for politeness' sake, from time to time.