Currently the default en-US dictionary used by spellchecker is using a legacy quirk to prevent it flagging other non-Latin languages as misspelled. The dictionary file was encoded in ISO-8859-1 and marked as such. Then the spellchecker will skip feeding words outside ISO-8859-1 to the dictionary.
While the quirk works, it is unreasonable to ask every other dictionary maintainers to put their dictionaries in legacy encodings. There ought to be a proper way for dictionary maintainers to specify "hey, my dictionary only care words consist of characters from codepoint x to codepoint y". Right now the quirk is broken whenever one use dictionaries outside the default en-US.