Maybe missing "." in "Loading..": https://git.xfce.org/apps/ristretto/tree/ristretto.appdata.xml.in#n74 Since this string have 3 "."'s: https://git.xfce.org/apps/ristretto/tree/src/main_window.c#n1614 Perhaps use placeholder to make sure the strings are identical.
Igor referenced this bugreport in commit 7cfab432c6222dab52d70fe64ea795e321ec8790 appdata.xml: Remove releases info and update URLs https://git.xfce.org/apps/ristretto/commit?id=7cfab432c6222dab52d70fe64ea795e321ec8790
Thanks for the report! I've removed the releases info from appdata.xml since it was outdated, it's a pain to update and translate such kind of info, and I'm not sure whether it's useful at all.