I am preparing myself for an application (one has to speak German, Spanish and English). My Spanish is very good because it is my mother tongue and my German is good because I have been living here for almost 10 years now. Unfortunately my English is not as good.. I could ask you if I have some questions about English grammar, couldn`t I?
(If you want the tag "couldn't I" then your base should be "I could." If you just want a question, then keep it as "Could I" and delete the tag. Try very hard to not use ... as a catch-all punctuation device. I think you're wrong to say that your English is not good - perhaps only not as good.)