Spellchecker insists on UK English with no way to fix in either OS or Keybase Settings

Keybase ignores my spelling settings, and insists on spellchecking with UK English spelling rules, whether I specify US English or Canadian English.

Two examples from Keybase: "socialize" gives me a red underline, but "socialise" does not. Similarly, "airplane" has a red underline, but "aeroplane" does not.

I see no preference that manages this in Keybase.

I'm running Ubuntu 19.10.

I have no idea if this is based on my location (Canada) or if people located in the US are having the same problem.

Start by reproducing the spellchecking behavior on Ubuntu 19.10, testing the reported “socialize” and “airplane” examples with US and Canadian English settings. Trace where Keybase obtains its language preference; done means the selected spelling rules are honored or a clear Keybase setting is provided.

Written by the indexing model from the issue text.