If your language is not int the list, you can try to translate it through; https://weblate.freepbx.org/
You can create your language and translate modules.
If the module is trnaslated, then it should be included in the modules.
Check /var/www/html/admin/modules/A_module/i18n/
If your language is here, then maybe the issue comes from the O.S.
FreePBX 16 (Install from Distro or from another O.S?).
Our Weblate version , the API given the language code cs for Czech, but this locale is not available on the system. Instead, the system has cs_CZ available.
Latest Weblate has support for this and we have plan to fix this but can not provide you exact ETA as of now.