The SitePoint PHP blog has a post from Roland Clemenceau about localizing the TinyMCE editor, a web-based Javascript HTML WYSIWYG editor. It's dropped in to make textareas rich-text editing components of your page quickly and easily.

This tutorial assumes you have built a PHP multilingual site/framework and you, or whoever the admin is, would like TinyMCE to automatically switch to the current language of your site. [...] Although TinyMCE is a great wysiwyg editor that provides localized files for almost every known language, it still does not come with an automatic language-switching feature. It's very easy to roll your own solution to this problem, though, so that's what we're going to do now.

He shows how to use extra language files in coordination with a "language" setting. This setting (usually something like "en" or "es") can then be set as a configuration option when the TinyMCE instance is added to the page.