@MalcolmB you are to be commended, your heart is in the right place.
Unfortunately this is doomed to failure - as @Smokin Joe points out no Thais will understand you. Really. You are going to stress yourself out for nothing; I tried this and in the first sentence or two I went off-road and it was trash from that point on.
Your girlfriend most likely has a Thai friend with decent English language (preferably speaking and reading). Let her give it. Spend an evening going over this with her (you may have to pay a bar fine) Damn, I wish I had done this ...