You can add new language in ‘System/ Localization/ Languages’ menu. Then you can use button “cloud with down-arrow” to download translations for your language from Crowdin (if it available).
If you want to have changes in that language – use ‘System/ Localization/ Translations’.
After changes you can apply language in System configuration settings.
Please, note that Ui of OroCRM translated only by ‘System/ Localization/ Languages’ and ‘System/ Localization/ Translations’. Localizations does not affect UI translations now.
So, please use change settings related to Language.