Here is a very useful tool if you want to pinpoint a character’s regional or foreign accent in English.
If you have an insatiable curiosity for English words, have fun with the Corpus of Contemporary American English. Type in any English word, and you can view how it has been used in speech,fiction, popular magazines, and many other genres from 1990 to present time.
The Oxford English Dictionary Corpus has a collection of 37 million words from Old English to the present.
There is also the British National Corpus for British English.
For other languages, there is a Corpus del Espanol and Corpus de Portugues.