Description

While working on #2388 i noticed that when we have an "explicit id" and extract with po formatter, the information about original message is not stored anywhere. So if those PO files than imported into some other systems it's hard to figure out what source text associated with this key.

In general, po files is not designed to work with "key" approach. Current implementation when key printed into msgid
(by the po spec msgid should contain a source message) is a workaround and kept historically during rewrite between v3-v4.

I have better idea how to do this, but any changes in this matter requires migration of catalogs and would be a big disruption - but i still have this on my plans.

So in this PR, source message would be printed to source locale and template catalog into the translation.

Before (after extraction):

en.po

        #. js-lingui-explicit-id
        msgid "custom.id"
        msgstr ""

After change in this PR:

en.po

        #. js-lingui-explicit-id
        msgid "custom.id"
        msgstr "with explicit id" <!-- source message printed

So printing original message to translation is also not strictly following the spec, but also not breaking it.

Also this PR adds more sanity to the PO Formatters, it cleans up after itself the comments used as serialization flags, such as js-lingui-explicit-id they make sense only for formatter itself, and should not be propagated to the lingui internal data model.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Examples update

Fixes # (issue)

Checklist

• I have read the CONTRIBUTING and CODE_OF_CONDUCT docs
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)