I'm just resurrecting my own thread here...
The spellchecker language under the Text tab will "randomly" change itself back to US, instead of staying on British, resulting in "colour", "optimise" etc being listed as wrong. Has anyone else run into this issue?
... it even changed itself while I was typing this post! 😱
Edit: I just found that you can choose "Setup" from the language list, which gives a list of all languages where you can rearrange and disable them. I've moved British to the top and disabled US. Let's see whether that works.