Where does the spell checking function comes from

One way to have your "dictionary" include spellings from other languages is to translate the document into the language you desire. This will create a copy of the document in the language you choose.

Though this way may produce some weird results, I find that once translated the "spell check" does provide corrections with the translated language, though it will also still correct English spellings.

An example of one weird result I've seen while translating from an English document to a German one had Sprechen sie deutsch translated to Do you speak English. Though, it appears this may have happened because the document had been translated multiple times between languages before translating it back to English, where I added the line which spell check corrected it for German spelling.

Another weird result was the translation failing to separate words causing the "spell check" to flag the word, but it did give the correction in the language it was translated to.

Hope this helps.

Spell check can be accessed from the top navigation bar on Outlook. After you click "Spell check," all misspelled words in your message body will be underlined in red. You can then left-click the word to get a pop-up list of corrections.

The screenshot above was taken in Internet Explorer 9.

I noticed that in Firefox, the Spell check feature is absent from the top navigation bar. This Microsoft Community post seems to corroborate that experience, explaining that browsers that have built-in Spell check will not display this feature in Outlook.

Outlook.com has a Manual Spell check button on the top next to Options.

It does not appear unless you are writing an email.

Note that the Spell Checker does not work in Mozilla Firefox, Safari, Google Chrome and possibly other browsers that do themselves check the spelling of text you type.