Who decides what is "proper grammar"? Who decides when to make changes when the grammar of the people change, and why is it important to follow "proper grammar" if languages change so often?
Edit: to clarify my question, I'm asking specifically about parts of language that can change over times, such as semantics and phonetics, and not about the grammar rules that are consistent across all languages
"Proper grammar" is a common term for what is actually better described as "standard" usage. The point of having standard usage is to optimize mutual intelligibility; having a consensus on vocabulary, pronunciation, and grammar helps prevent misunderstanding. There will always be variations across a population and over time, but striving for standard usage (or "proper grammar") does not seem like a bad thing.
Ultimately, it is the collective speakers of the language who determine what is proper. Linguists try to capture language changes as close to real-time as possible. Textbook writers are more conservative.
For the record, there are very few (if any) "grammar rules" that are consistent across all languages. There are thousands of languages, and some of them (such as some of the Australian native languages) would probably seem to you to violate every rule you might think to be universal.
As already noted, proper grammar is the standard usage of the language. However, it bears clarification that proper grammar (as well as vocabulary) is largely regional. What is correct in American English may not be correct in Canadian, British, or Australian variants of the language, but even among those variations, there are further dialects such as Yorkshire (examples of which may be seen in Wuthering Heights) and Queen's English (which is the posh accent many foreigners associate with British English, despite being spoken by an estimated 2% of the population).
What should be considered proper grammar varies depends on the intended audience. Speaking in the dialect commonly known as ebonics is fine among those who speak it, but may paint the speaker as uneducated outside of that crowd. The inverse is also true; speaking in an dialect that is generally associated with higher income levels, social status, or education may offer the impression that the speaker is arrogant when used outside of the native speakers of the dialect.
Ultimately, proper grammar is decided by usage within a community, and what is proper in one setting may not be correct in every setting. One should include a 'u' in words such as colour or honour when writing to a British audience, unless the writer desires to portray oneself as an American. Unfortunately, it can be nearly impossible to communicate many complex ideas without choosing some dialect and its associated connotations, as the simplified form of English (Basic English) contains only 850 words.
Outside of English, many other languages are standardized. French is a notable example; France employs a Minister of Culture to monitor their language and prevent intrusions that are not deemed sufficiently Gallic; Quebec does the same with their Quebec Board of the French Language. Languages such as Hungarian, Kurdish, and Tibetan have had similar such attempts at standardization. Only time will tell if increasing globalization creates such similar institutions in charge of English, but as of yet, the closest we have are the style guides and the school system.
