There is a language picker option that allows users to choose their language at login. The languages listed are those active in the instance, each with the relative internationalization plugin dictionary installed.
It is not possible to hide an active language from the picker. If a language is enabled, users see a choice list on the login page to select their language. If the language is deactivated from the instance translations, it won't be available for any user.
Alternatively, an administrator can hide the language picker completely by setting the system property glide.ui.language_picker.enabled to false.