I’m using Bedrock 1.6.3 with WP 4.5.2 and I am trying to translate my wordpress plugin bud I can’t get it work.
The plugin is stored in web/app/mu-plugins/my-plug-in/my-plug-in.php and is working. The strings to translate are wrapped in the __('str', 'text-domain') and _e('str', 'text-domain') function. The necessary .mo and .po files are created, named like my-plug-in-de_DE.po and stored in web/app/mu-plugins/my-plug-in/languages.
Have you tried localizing your plugin with the localization app POEditor?
I found it makes translating strings from .po files easier, especially if used with its translation management plugin (to be found on WordPress here).