For the impatient (<abbr title="Too long; didn't read">tl;dr</abbr>): <ol> Open <i>System Preferences</i> Select the <i>Keyboard</i> panel Activate the <i>Text</i> page Show the drop-down under <i>Spelling:</i> Select the last item in the list, named <i>Set Up...</i> Check/uncheck to select and drag & drop to sort your preferred languages </ol> <h>Details and Screenshots</h> I recently set up a new Mac to work on develop iOS apps at <a href="http://encodo.com">Encodo</a>. I relatively quickly noticed that the machine was set up for British English (en-GB) rather than my customary US English (en-US). There aren't many differences, and I could have lived with the display language in en-GB, but the spellchecker was driving me nuts. <img attachment="choose_main_languages.png" align="left" caption="Choose Main Languages">First, I fired up the <i>Language and Region</i> preferences panel in the <i>System Preferences</i>. I removed British English and installed US English instead. Some applications updated immediately and the others updated after I'd logged out once and logged back in. <img attachment="opera_language_preferences.png" align="left-column" caption="Opera Language Preferences">But the spell-checker was unaffected. I do a lot of writing in the Opera browser, which also has standard language settings. As far as I know, those sent to web servers as part of requests to indicate which languages are "acceptable". I noticed that here, too, British English had been installed (presumably matched to the prior system language). I switch to US English but was disappointed to see that these language settings didn't control the spell-checker in the browser. <img attachment="set_up_spell-checking_languages.png" align="left-column" caption="Set Up Spell-checking Languages">So where was Opera getting its spell-checking dictionary? From the OS? But I'd already told the OS to use US English, hadn't I? Or had I? Just because the OS is using US English to <i>display</i> text doesn't mean it's using it to <i>proof</i> text. Fine. I gave up temporarily, resigned to ignoring red. wavy lines for missing "u"s in "color" and "favorite" and for improper "z"s where "s"s were expected in "synchronize" and "personalize". <img attachment="choose_spell-checking_languages.png" align="left-column" caption="Choose Spell-checking Languages">Just today, though, I finally searched for "MacOS change spell-checking language"---rather than searching for "Opera change spell-checking language"---and hit paydirt. The settings for which spellcheckers to use in MacOS are found not in <i>Language and Region</i> but in <i>Keyboard</i> => <i>Text</i>. Naturally. From there, you can enable/disable and change the order of the available spell-checking languages not by clicking a button labeled <kbd>Set Up...</kbd> but by selecting the last item in the drop-down under <i>Spelling</i>.