Poor use of the radio is not necessarily an indicator of someone being a "bad pilot"! There are plenty of pilots with a limited grasp of English who probably have flying skills that would make 95% of the world's pilots jealous.
Having said that, a limited grasp of English can also combine with poor airmanship. So, keep it simple! One instruction per transmission, speak slowly and clearly, and be prepared to lose a little expedition for however long it takes to get that particular aircraft onto someone else's frequency. DON'T GET ANGRY!
I've only heard one bit of swearing on the radio in 14 years or so - a British pilot asked for push and start, and I had to tell him that his CTOT had just changed and was now over an hour away. "Oh sh*t", he said. It seem justified to me, so I ignored the language and put a ready message into the system for him.
We have to be aware that what seems annoying to us in ATC, can often be due to factors we don't appreciate. The 777, for example, has automated cockpit announcements for just about everything and they can make it difficult for the crew to hear us. As per the law of sod, this is most likely to happen when we're issuing a turn instruction that needs to happen RFN.
The things that annoy me the most are:
1. a pilot coming onto the frequency without listening out for a few seconds first, and thus stepping on a readback.
2. a pilot being told "standby" but going ahead with his life story anyway, and
3. [and this is really not that important but hey, they have the job they always wanted!] grumpy pilots.