Spell checking in comments and posts is done by your browser and usually this is set by default on the browser. However, in some cases you may need to enable it or may wish to change the language for spell checking on the browser.
Depending on the browser, you can usually find the spell check feature under "Advanced Settings" in the Settings or Options menu.
For example, in Google Chrome you can enable spell check by clicking on the three dots in the upper corner and going to Settings > Languages > Spell check.
For Microsoft Edge, you can find the spell check options by clicking on the three dots in the upper corner of the browser and going to Settings > Languages > Use writing assistance.