I installed zh_TW (LANG_TW) language pack from www/firefox-i18n port. However, Firefox refused to load it and the following message was showed on about:addons page: Traditional Chinese (zh-TW) Language Pack could not be verified for use in Firefox and has been disabled. If I manually downloaded the language pack from https://addons.mozilla.org/en-US/firefox/addon/traditional-chinese-zh-tw-l/, it could work properly.
I can confirm this bug with the german translation. With www/firefox-63.0.1,1 and the corresponding i18n package on Freebsd-12.0-BETA3.
grahamperrin@momh167-gjp4-8570p:~ % date ; uname -v Sun Dec 23 11:50:37 GMT 2018 FreeBSD 13.0-CURRENT r342020 GENERIC grahamperrin@momh167-gjp4-8570p:~ % pkg query '%o %v %R' firefox firefox-i18n xpi-quick-locale-switcher www/firefox 64.0_3,1 FreeBSD www/firefox-i18n 64.0_2 FreeBSD www/xpi-quick-locale-switcher 1.7.8.5 FreeBSD grahamperrin@momh167-gjp4-8570p:~ % I see no way to switch locale. NB bug 221916 I toyed with manual installation of the Français Language Pack <https://addons.mozilla.org/addon/fran%C3%A7ais-language-pack/> – no difference, as far as I can tell, so I disabled this extension.
(In reply to Graham Perrin from comment #2) You can switch locale by setting intl.locale.requested in about:config page.
Firefox 65+ has native UI to select and download language packs. Please, use that. www/firefox-i18n is gone after ports r497194. https://support.mozilla.org/kb/use-firefox-another-language (the page is translated into many languages)