I don't know about WYSIWYG editors, but I know that Bluefish does a great job of using multiple languages in the same file. (Though, if you are the developer, look into better code separation so you don't have to worry about it).
the problem is its an old site, and now there are no resources to rewrite it, but we need to add some localization, and that is implemented in that way. php files with some html , and the interpreters are struggling with the html. that is the problem.any other solution??except rewriting the whole thing?
Have you tried Bluefish? You may have some problems finding a WYSIWYG editor that matches beautifully because rarely do they actually produce beautiful code either. However, you should be able to add these tools without a total rewrite; you may just have to use a text editor rather than a drag-n-click.