Rated 1 out of 5 stars

is not steady: sometimes it remembers my choices, sometimes not. And sometimes all the words are underlined in red although when I check the spelling, it is the correct one.

This review is for a previous version of the add-on (1.10.2). 

Hi, sad to hear that. It should be working. Which version are you using?

I released the version 1.10.2 yesterday with some fixes.
If you are in an old one, you will be updated automatically eventually. If that's not the case you can download that version from mozilla and install it manually here: https://addons.mozilla.org/thunderbird/downloads/latest/automatic-dictionary-switching/type:attachment/addon-116388-latest.xpi?src=dp-btn-primary

Hope this helps.

If you are in last version 1.10.2 and it still does not work, please email me and I'll be pleased to help.