This is useful for people who write right-to-left sometimes, as is done in Hebrew, Arabic, Persian and other languages.
So far I only have a solution for Firefox users, but I strongly recommend the browser to anyone. If you don't like 'messing around' with the computer and refuse to use anything but Internet Explorer, you're probably beyond my help anyway. Wait for Google fix this themselves, or write in English.
So if you use Firefox, install the Clippings add-on from the Mozilla site: https://addons.mozilla.org/en-US/firefox/addon/1347
It's a good add-on. I recommend learning the basics of it for other uses as well.
Now download the Clippings export file I made from http://www.mediafire.com/?sharekey=950ac357b5088b54d2db6fb9a8902bda
It contains two clippings - 'RTL embed' and 'RTL end'. Each clipping is a single control character, U+202B and U+202C respectively, which start and terminate a region of RTL directionality.
To import the file into Clippings, right click any text-box in Firefox, select Clippings > Organize Clippings, in the dialogue click Options > Import, and select the file.
Your setup is now done. To switch to RTL in Gmail (or any other text-box in Firefox), simply right click and select Clippings > RTL embed. In most web pages, starting a new paragraph or aligning the text to the left will terminate the RTL region, but if you want to terminate it yourself you can right click and select Clippings > RTL end.
I've also defined shortcuts for the clippings (which you can change), but the shortcuts only work while input mode is English (once you start typing in Hebrew, for example, you have to switch back to English to use the shortcuts). Press ctrl+alt+v to enter Clippings' shortcut mode, and then 'v' for embed or 'f' for end.