Providing translations for custom modules
I had a similar problem that it didn't work but it turned out that my problem was with file permissions that weren't set correctly on the PO file I was trying to import.

I debugged it as follows:

On the following url admin/reports/translations you can see where it looks and what it looks for. (handy for debugging the tokens) Expand the status information for the available language.

Every time you change your info.yml file don't forget to rebuild cache and trigger the translation update manually.