Malika, that function is system-wide so you should be able to use it with yahoo e-mail. If you're running Windows Vista or Windows 7, there's a little tool on the Task Bar (the bar at the bottom of your screen with the clock, and several small icons). One of the icons looks like a miniature keyboard. You can use it to change the character set for your keyboard. The one difficulty is that if your keys are all labelled in English, you will have to memorize the equivalent French characters. I do know that the ? key changes to an acute accent, but don't know the others off-hand.
I think you can buy keyboard stickers that have the English and French character sets on them for when you switch between the languages.