One of the most important rules in interface design is that people don't read anything. So, no, people don't
read or follow instructions. Hackers are one of the (very) few cohorts of people who tend to expect people to read their (expletive) manuals, and even we don't read squat...
...Related posts about code tags, for example.
(Though I do understand that it is easy to lose posts that are a week old or older.)
I don't think the "we're in America, speak English" sentiment relates here. This is an English language forum. As such, it is not unreasonable to require people to at least attempt to use English.
The reason for this is that English is the de facto
international language on the internet. Here's a half-decent article I googled-up, as it were:
(BTW, his 'English is unsuitable' stance is straw horse crap -- all his reasons apply equally to any other national language, meaning he thinks a constructed language would be best.)
Many of us actually do speak multiple languages, and it is not uncommon for some OP to find someone to use his native language with him. Spanish, Greek, Cyrillic languages, Portugese, Chinese, are but a few of the languages common to our most prominent members.