New subtitle file translation is prepopulated with source language

[RobbyZ]

Describe the bug

Creating a new translation of a .ass subtitle file copies all source strings instead of starting blank.

This is possibly because 'Monolingual base language file' is required to be set for subtitle files?

To Reproduce the bug

1. Set up a .ass subtitle translation component
2. Create a new translation
3. Note that the new translation's strings are filled with duplicates of the original (fortunately are flagged as warning)

Screenshots

Additional Info

Also, just a note this does not appear to be a sub2po issue, as when sub2po is run standalone, it does not prepopulate msgstr:

#. visible for 3 seconds
#: 00:00:23.250--%3E00:00:26.940
msgid "If you pull this off, they're willing to let you come back."
msgstr ""