I thought there was one kind of speech, human speech!
The existence of two forms of speech maybe an indicator of the existence of another species, which the scripture calls the Jinn-beings.
I also believe that speech is an effective tool of control, for example in Egypt the politicians almost always use formal speech (which is standard Arabic) the majority of Egyptians can barely understand it! this keeps a calculated distance between the elite rulers and the subservient masses.
In Japan this takes a more extreme form, they actually have more than one style of speech! 1- the polite speech, 2- plain speech, 3- Respect language
Here is a definition i found in a Japanese language book:
1- Polite speech: is used in formal situations where participants interact rather like "representatives" engaging in "official business". Polite speech is used in business, in the class room, when addressing an audience, when speaking to a superior, shopping, or when speaking to a stranger on the street.
2- Plain speech: is used in informal, everyday situations between family, friends, equals, or when addressing inferiors such as children. Most thinking is done in plain speech.
3- Respect language: besides the use of polite speech, further respect may be shown by the use of honorofic terms when referring to others, and humble terms when referring to oneself.
The majority see this Japanese attitude as very healthy, however it is destructive in reality, since it widens the gap between the superiors and inferiors to the extent that the individual reveres his superior more than God thus makes him unable to question him, and all his actions will be calculated according to what his peers, superiors will think of him, this is a form of idol-worship the only unforgivable sin if maintained till death.
I think it has origins in the Samurai age when there was a caste system like the Indian one. Humans should interact respectfully using one style of speech, not too slang, nor too formal, this balance should be maintained since the language i think affects our attitudes and is used to make us subconciously submit blindly to authority.