"sorry for my bad english but i am german/french/chinese/greek/scottish(seriously, wtf?)..."
is the line one puts after a grammatically correct and understandable English sentence just to point out that he/she is not "English/American..."
Some dude: Hi, my name is *insert non-English name here* Sorry for my bad English but I am from *insert non-English-speaking country here*
Some other dude: I have been... *perfect speech*... and I genuinely apologise (emphasis on the "s") for my terrible English. However, I am...