When different users seek help for their problems, it is critical that proper English be used. The reason for this is it is often not very clear what a user is asking when they post a question on a forum or ask in IRC. If the question isn't clear, a clear answer or no answer can really be given. Instead those users that are willing to help other users have to ask questions of clarification that make the process wearisome and unpleasant. With open source software you don't want to make the people supporting feel like it is drudgery to do so.
Most of what I have seen probably has more to do with foreigners who have not gained proficiency with the English language than anything else. At the same time though I have seen this problem with people whose native tongue is English. Unfortunately for other foreigners English has become the default language for anything to do with a computer. To a great extent that has to do with the fact that America has made a good number of contributions to computer science from programming languages and operating systems to establishing the framework of the internet. To get help and become proficient with a computer you need to have a mastery of English. This applies for reading documentation as well as asking questions.
The greater portion of questions I see are development related as that is my chief interest these days. Most new programmers when they ask their questions about how to hack in a given programming language (normally C) can't even hack together a good sentence in English. Being able to hack in C involves being able to hack in English. The programmers I tend to deal with that have achieved a mastery of C have at the same time demonstrated a mastery of English. Some of them are lazy with using that mastery, and others function as grammar nazis with what you write. Knowing English is key to obtaining knowledge of your desired technical field.
