apparently, the PO generator treats underscore as something important, it assumes it's the GetText underscore method, thus, breaking things in a spactacular way. removing it solves it.
Related issues
Bug #13567: locale:pack failure in facets/managed_host_extensions.rb
Added by Tom Caspy about 8 years ago
fixes #13567 - fixing locale:pack
apparently, the PO generator treats underscore as something important,
it assumes it's the GetText underscore method, thus, breaking
things in a spactacular way.
removing it solves it.