Is there a way to filter or sort my iOS Apps by Account which was used to purchase them?
I've multiple accounts as well. I'd like to use free US apps as well that they don't provide on my countries App Store. Neat trick.

Anyway. Unfortunately there isn't a sort option existing in iTunes like the one you've asked since Apple won't care about our situation in general. Most users will only have a single account.

Also there isn't a third-party app for that.

So we're stuck with changing accounts all the time. But one advice that I can give you is that check some of those apps that you've downloaded with your US account in Austria store. Maybe those developers placed theirs apps in Austria store as well as of now. Deleting the old one and re-downloading it will keep your app library more organized.