Clever idea using flags! Looking at the docs of gettext and it doesn't seem like there's a specific syntax defined but most of them are in kebab-case with the exception of the range flag. I guess priority: low would be an option too. But here I think we should just go with low-priority.

Having some tests would be nice and shouldn't be that hard to write. Here's the example for fuzzy: https://github.com/GlotPress/GlotPress-WP/blob/fae8e0035385f1efd7248db604aed692b46f70ba/tests/phpunit/testcases/tests_things/test_thing_translation_set.php#L33-L47

I guess priority: low would be an option too. But here I think we should just go with low-priority.

While priority: hidden/priority: normal would read better than hidden-priority/normal-priority I expect the readability of low-priority/high-priority is more worthwhile here, so might as well just go with that.

I had read a few gettext pages for the flags field, but I didn't spot anything that suggested a valid set of characters, only that it split by ,. I got lost trying to find it in the sourcecode.

PR updated, and a test included, even if I did write it without running it locally.. with a PHP Syntax error originally.. :)

While priority: hidden/priority: normal would read better than hidden-priority/normal-priority

Argh, naming is hard… Since it seems using a colon is fine maybe go with that version?
I'm now even wondering if we should prefix the custom flag with gp-? As in gp-priority: hidden, gp-priority: normal, gp-priority: low, and gp-priority: high.

Argh, naming is hard…

Tell me about it...

I'm now even wondering if we should prefix the custom flag with gp-

I too had thought about that... I ultimately went with not-prefixed so that it would be a more natural flag in exported files in other tools, Seeing low-priority there would be more relevant than gp-.....

But I also don't know how 3rd party tools will process flags, or if they even will display it? seems like it's more likely that other tools simply won't do anything when it see's these flags, and if it'll never be displayed, it doesn't matter if it's readable or not.. just concise/easy to parse/accurate.

gp-priority: hidden, gp-priority: low, gp-priority: normal, gp-priority: high works for me though, and is just as good as the other options, if something displays it, or parses it, it should be obvious that a) it's a custom GlotPress thing and b) it's a priority field and a value.

I had also thought about cramming the priority into the Comment section for that purpose, but ultimately decided that using the "internal" translation flags seemed more appropriate than a human-readable area..
It would've been compatible with other translation formats going that route, but it might just be that .po support is the only format that currently implements flags.