We have code for migrating the single-language spellcheck preference to the multi-language spellcheck preference.[1] This has been in place for 2 years. We should eventually (soon?) be able to remove this code.

[1] https://cs.chromium.org/chromium/src/chrome/browser/spellchecker/spellcheck_service.cc?rcl=9b173f861a75fa70a39515bcca7d323684d1cfe0&l=67

assigning to Rouslan for triage.