I installed this language package: https://packagist.org/packages/magento2translations/language_bg_bg with composer, but it isn't applied to the frontend. I've cleaned the cache, I can see the files in vendor.
In the backend I changed the language for my user to bg and some of the strings in backend are translated. In the frontend however everything is set to english. I've selected the language pack as the locale, so english shouldn't be shown in frontend.
Any ideas what I might be doing wrong?
Instead of adding your language pack to vendor directory - add language pack to app/i18n directory.
Below is the detailed instructions step to install Bulgarian(bg_BG) language.
For More Details refer this link - https://github.com/denchev/magento2-bulgarian-language-pack
Note : While doing this all make sure you are in developer mode - as magento 2.2.2 version you can not change locale while your in production mode.