Feature request: Add option for Extract to write a default msgstr

[MrBillium]

Current behavior is for Extract to write msgid + empty msgstr to a .po file for user to fill in later with translation. I would like msgstr to optionally contain <langcode> - msgid.

Why do this?
Chances are the person who does the translation is not the developer and is not familiar with which fields are supposed to be translated. By actually setting the msgstr to something, the translator can spot it displayed on the site in context and act on it

I did this myself with some janky Python code to modify the .po file after creation if msgstr is empty but it would be much nicer & safer to do during creation.

Here is some sample output for a french file ( first one was fixed by a human, rest of them are still to-do):

`#: templates/admin_bp/customer_edit.html:19 msgid "Name:" msgstr "Nom:"

#: templates/admin_bp/customer_edit.html:20 msgid "Description:" msgstr "fr-Description:"

#: templates/auth_bp/login.html:46 msgid "Username" msgstr "fr-Username"`