CLI replaces `Language` value with source language in PO files

[ceolinwill]

What's happening?

When running lingo.dev run to generate new translations for PO files, it replaces the "Language: en\n" value at the top of the file with the source language instead of keeping the target language value.

Exact Reproduction Steps

1. Create a project with two PO files: en.po and es.po
2. Run lingo.dev run to translate strings for es.po

You can also see this behavior in this repo's demo where the Language header for the Spanish translation is set to en.

Expected

It shouldn't change the "Language: es\n" header

Actual

It's changing the Language header from es (target) to en (source)

Screenshots/Videos

lingo_po_bug.mov

[vrcprl]

Nice catch, thanks for flagging this! Reviewing the PR.