Thanks, I spotted that while translating and made this fix for the next release.
Did I understand you correctly that the standard is capitalization like this? Also, do you know where to find documentation of stylistic things like this?
Well. We’ve got our style guide for Swedish at https://wpsv.se/engagera-dig/stilguide-for-oversattningar/
I don’t think there’s any WordPress style guide for US English though. After all, nobody translates TO en_US here, only FROM. Especially in titles/headers they tend to capitalize all important words. And then the en_GB team teaches their contributors to use “sentence case” instead.
So you can go both ways. But I think it’s totally OK to commit what you just showed.
It seems like WordPress, US English, uses title casing for most labels and menu items in the admin pages, albeit not coherently. The strings for the default post types ‘post’ and ‘page’, corresponding to the ones you pointed out, are consistently using title case. So I’ll use that in the plugin too.