is this really necessary? Montserrat is a font found on Google Fonts that can be easily imported with @import or just downloaded and then uploaded to your wiki (either as TTF, OTF, WOFF, WOFF2) for use with font-face.
The extension was developed for Wikimania 2018 (see https://phabricator.wikimedia.org/T194577) and has not seen development since then. While the code does not need much maintenance IMO, I am declining this per what @Lakelimbo said above. Sorry for the inconvenience.