Type about:preferences#content<enter> in the address bar.

On the bottom of the page, find Languages. To its right, press the Choose button. Make sure the correct language and district are selected.

Language Packs https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

You can check the spellchecker.dictionary pref on the about:config page.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

See also https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

FredMcD said

<strike>Type about:preferences#content<enter> in the address bar. On the bottom of the page, find Languages. To its right, press the Choose button. Make sure the correct language and district are selected. Language Packs https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack</strike>

FredMcD, this was about the inline spellchecker and not the language of ui or pages.