@valpackett ok i just looked into it and it seems that it is, in fact, editable from the wiki itself. so i was completely wrong about this :p

a fun thing about mediawiki UI strings is that they can be modified from the wiki itself, e.g. the default message placed on talk pages of new accounts is at https://wiki.postmarketos.org/wiki/MediaWiki:Confirmaccount-welc. and it turns out this seems to work for all translatable strings, the phone number text is at https://wiki.postmarketos.org/wiki/MediaWiki:Requestaccount-ext-text (and i will modify it promptly)