In Spanish there is an entity, the Royal Spanish Academy (Real Academia Española) which states what is a valid spanish word or not. In Japanese, I've read the Japanese Ministry of Education has an official list for joyo Kanjis, which is the list of kanjis people has to know and that if a newspaper, or if someone wants to use a kanji outside that list, it has to write its hiragana over it so people can read it. How about loanwords written in katakana? Can any word be adapted on the fly from its original language and be valid for being used anywhere (newspaper, document, contract, whatever), or is there a list somewhere written by the JME or any other entity about what is an official valid loanword for japanese?
There is no such regulation. But there is a list of "suggestions" (not even a guideline) from researchers in a government agency that tries to replace difficult loanwords with kanji words. It has no legal binding force. And there are some style guidelines which try to standardize spellings of loanwords.
For details, please see:
The first appendix of "The Dictionary of Intermediate Japanese Grammar" is dedicated entirely on rules on how to convert English words to katakana. It shows 18 rules, from page 615 to page 625, all of them based on pronunciation.
The Ministry of Education has published the following recommendations: http://www.mext.go.jp/b_menu/hakusho/nc/k19910628002/k19910628002.html