There's nothing wrong with the .NET resource system with culture-specific satellite assemblies etc as such. The bigger problem I see is lack of tooling and services for translation by non-developers, volunteers, professional translation bureaus etc. You can't expect them to fire up Visual Studio, clone your repo and start editing resx files. Especially as there's not even a translation view! With .po files, there's a ton of editors, online services, translators etc available.
With Windows Forms, at least back in the day, you could open a Form in the designer, change its language to something else and just edit text on the controls (or images, etc.). The changes would go into a resx file for the selected language. At first glance this looked quite cool, but on the other hand, translators now can ruin your UI, as there's not really a dedicated translation view that only allows to change resources ...
At a previous job we utilized https://webtranslateit.com/ - it organized everything easily for volunteers to provide translations for all resx strings.
Nice, but perhaps a little pricy for small startups. That's actually one of the things I didn't emphasize perhaps. Not only is the gettext/.po eco system big, there's also a lot of free or relatively low cost services available.
(With the new "source generator" work it should be relatively straightforward to implement a gettext-style source scan if that's what you want.)