Despite isv-Latn and isv-Cyrl passing all localisation tests on https://codelookup.toolforge.org/, isv is not. The Interslavic Wikipedia community has opted to use Latin as the default script. Upon creation of relevant files, including but probably not limited to MessagesIsv.php, isv should therefore have isv-Latn as the first fallback choice, and isv-Cyrl as the second. Furthermore, add isv in addition to isv-latn and isv-cyrl where needed, such as Names.php.

Update 1: A new discussion on script usage within Interslavic-language Wikimedia projects has been opened here.