I wouldn't mind a non-invasive in-line spellchecker, much like here in this comment box that Firefox is providing me. "spellchecker" isn't a known word, so it underlines it red. Many applications use such an approach.
I think user locale, options to change or include multiple locales, wouldn't encumber many users. If "lmao" was underlined, big deal? It's NOT a word or an abbreviation for 99% of dictionaries. Solution? Allow users to load standards following dictionary databases, override words, use a priority ranking system when using multiple databases, and etc.
I, however, wouldn't like an in-line spellchecker that offered suggestions! But if I'm spelling "separate" as "seperate" I certainly wouldn't mind it underlining the unrecognized word.