Automaticly select right Language for Firefox spell check

I am writing a lot in German and English so I have to change the spell checking language of Firefox constantly. I found a experimental plugin that is which tries to do this based on the webpage your one but so for it doesn't do anything for me.
There are several bugs opened for this issue with different ideas how to solve the problem.

• Based on the text you typed in the field so far
• Based on the HTML page lang tag

Oh, there is another plugin. It tries to detect the the language automaticly but you also can save your settings per page. Auto detect does not look good so far but it works for me.