I agree with Tom absolutely. Nobody will take you seriously if you use these words. Yes, they're fine in song lyrics and occasional short informal texts. Cartoon caption and subtitle writers also use them to represent fast, casual and informal speech. These are the only situations when they should be used. If you look through the pages of italki, you will see that native English speakers do not use these forms when they are writing. By the way, the word 'Englishmen' refers only to adult males from England, which of one of the four countries that makes up the UK. If you want to include all the English-speaking men - and women - from the USA, Canada, Australia, New Zealand, S. Africa, Ireland, Scotland and Wales, then it would be better to say 'English speakers' or 'native English speakers'.

Because people here do not approve of the use of those words. They are fine to say and type in certain situations, but you should have a good understanding of how people will view your use of language by using these words. Therefore, it is generally better to just not use these words at all. At least not until you reach a high level in English, but at which point you probably won't wish to use them anyway. If you want to use these words in general conversation, then go ahead, but this is a language site and people will assume you want to learn proper English which is why they correct you. If you wanted to write in the style of song lyrics and you told people that, people wouldn't correct these words. Basically, view these words as very informal.

I would say it is because this level of informality tends to be associated with children. Adults that speak this way tend to be viewed as being juvenile or uncivilized. In writing, this simply looks terrible.

You obviously are not sufficiently sensitive to differences in "register" (correct/incorrect, good/bad, formal/informal) in English. I'm afraid I don't know Russian but I am sure the same things are true in Russian. Can't you think of ANY phrases or words that you use every day, but that are not "good Russian," that you would correct if you heard a Russian learner using them? If I may be vulgar, using "gotta" or "gonna" or "wanna" is like flatulence. Everyone does it, it is perfectly natural, but there are times and places where it is unacceptably rude. And there is another problem. Foreign speakers seem to think that using these words will make them sound better, more natural and more "native." But they usually do not pronounce them correctly. The spellings "gonna," "gotta," "wanna" do not convey the sound accurately. The way you probably say "gonna" is not the way a native speaker says it. What I say, for example, is somewhere in between "gonna," "goyna," and "goinga" but not like any of those three. When a foreign speaker says them, it does not sound natural, it sounds affect and phony: I am trying to sound like a native and failing.