Maki Pale Moon does not currently support Intl.DisplayNames. There’s a workaround in the forum: https://forum.palemoon.org/viewtopic.php?f=70&t=32788#p266223
Mastodon should include a polyfill for it instead. DisplayNames was introduced in 2021 which is too recent for a standard that adds features every year (and a lot of them are simply syntactic sugar that can be done in previous editions anyway. A great way to gatekeep smaller browsers from ever becoming fully “standards compliant”!). 10 years ago should be the baseline, which means Standard ECMA-402 2nd Edition (June 2015).