Blackboard Learn is a fully internationalized application supporting several localizations out of the box, plus supports additional localizations via Language Packs. For a Building Block to be fully integrated into the platform, it's a good idea to internationalize and localize your plugin. This page will not go into the details of localizing your plugins (that's covered in the developer documentation) but will cover some quick gotchas to know when localizing.
Without a default locale set for your Building Block, the platform will assume the plugin is not localized and will not translate anything. Make sure your bb-manifest.xml file includes this as a subelement to the plugin.