Bug persisits through 12.0 [CASE:4331923]
<...> does Mma actually recognize Euclidian (vs Euclidean) as a correct word, which just happened in my v.12.
Experimenting with the spell checking options and this word revealed that it seems to be a "protected" word which can't be touched by Mathematica's spellchecker independently on the language settings and other options. The most bright evidence is that setting the language of spell checking to Russian doesn't affect this word: now all usual English words are marked as misspelled including the word "Euclidean" but excepting the word "Euclidian". Try:
CurrentValue[$FrontEndSession, DefaultNaturalLanguage] = "Russian"; CellPrint[TextCell["Euclidean Euclidian русский текст ddd dddd ", "Text"]]
It was a very surprising discovery. What could be a reason for this? Are there other "protected from spellchecker" words in Mathematica?