TinyMCE uses simple text files to localize. There are two places you need to know when localizing Tiny: The general language directory and the specific language directory per plugin. You can look for both of them in:
General Language Directory
Plugin Language Directory
Creating a New Localization file
The easiest way is to copy the localization file for English language and edit it. Use the same file name convention as for the other languages, that is by using the ISO code for the language.
Edit the new file with your favorite text editor and save it.