"I'm not sure this is actually directly an issue of ... grammatically correct English but never say "please" and "thank you"."
"But would it still be proper English? I think that there is more to speaking a language than just the vocabulary and grammar. The way the language is commonly used is equally important (and probably the most difficult to learn)."
I don't speak German, but I can manage a little French, so I'll assume for the sake of argument that the same applies in both. When I'm speaking French, I don't notice that I'm saying "S'il vous plait" and "Merci" any less than I say "Please" and "Thank you" in English. It may be that I do so more often than a native French speaker would, and therefore appear excessively polite, but it's not something I was aware of until you brought it up.
However, I can't speak French without being aware of the "tu/vous" issue, so it seems to me this is far more central to the French language, and so the fact that French has this special polite form which you have to use in many circumstances suggests to me that it is a more polite language than English.
"Probably a good idea. Although the "taboo" words have lost ... one would not use in a polite or formal context."
"I still don't see the point in doing this. What is the advantage?"
I work as a university lecturer, and many of the students I teach have come from overseas and speak English as a second language. I've often seen these students using inappropriate colloquialisms in what is meant to be formal written English. The reason is they haven't had enough experience to realise that these forms that they have heard spoken just don't look right when put in a piece of formal documentation. For example, they may have heard "gonna" used to form the future tense in spoken English or in pop song lyrics, and from that may assume it is the standard way to form the future tense. So occasionally I see it used in their exam scripts and project documentation. It looks bad an unprofessional, and would certainly hold them back if they were to do that when in employment.
Exactly the same might apply to the "four letter" taboo words. Someone who hears them used casually in normal conversation, which in many contexts isn't uncommon in England, might go on to use them in contexts where it would be considered rude or unprofessional.
"Don't other language have similar taboo words, usually conneted with sexual or excretory functions?"
"There are taboo words, but IMHO they are not as taboo as here. I'm always surprised when I hear a (BBC) radio presenter apologising for bad language - people would use much worse words than f*** or c*** in German without apologising, even during day-time."
English, unlike German, has developed a vocabulary of Latin-derived words, which often parallel existing Germanic-derived words. In some cases the Latin derived word is the one used in formal contexts. I guess that as German did not develop this parallel vocabulary, the Germanic derived word remains used throughout.
"These 'taboo' words are something weird for a foreigner as there is no cultural background telling that these words are ... said group made me very conscious of this kind of language, so I think my language shouldn't be too bad."
The idea of regarding these words as completely unpronounceable is now regarded as somewhat old-fashioned. There was once a time when men might use them in conversation between themselves but regard it as bad to use them in a woman's presence. That no longer seems to be the case.
""Thou" or related second person singular forms survives in some dialects of English."
"In which dialects?"
Yorkshire in particular is known as a place where "thee", "thou" and "thy" (all sometimes pronounced or written "tha") survive in the everyday speech of ordinary people, and in some other northern parts. It also survived in some south-west dialects such as Somerset, although these have largely died out.
Matthew Huntbach