Pidgin is a cross-platform instant messenger, that speaks a lot of different protocols.
It is not uncommon that you use different languages in different chats. While Pidgin has an out of-the-box spell checker, it unfortunately does not remember the last selected language.
To add this feature you need the “Switch Spell” plug-in.
Under Debian you can install this with
sudo apt install pidgin-plugin-pack
Don't forget to enable the plug-in under: