If you send messages in more than one language, you can install additional spell-checking dictionaries to Thunderbird. After installing a dictionary, it is easy to switch from one language to another.
Language dictionaries are available as Thunderbird add-ons. See the Thunderbird add-ons - frequently asked questions for more information.
That's it! Your new dictionary is installed. To uninstall it, open the Add-ons Manager, find the dictionary on the Dictionaries or Extensions panel and click the Remove button next to it.
Right-clickHold down the Ctrl key while you click in the message you are writing and select a language in the Languages menu.