Several translatewiki projects uses $1, $2, $3, etc. as a parameter format. They should all use the same Insertable class, and it should be in the Translate extension rather than in the translatewiki repo.
Several [[ https://www.mediawiki.org/wiki/Translatewiki.net | translatewiki ]] projects use $1, $2, $3, etc. as a parameter format. They should all use the same Insertable class, and it should be in the [[ https://www.mediawiki.org/wiki/Extension:Translate | Translate extension ]] rather than in the [[ https://phabricator.wikimedia.org/diffusion/GTWN/ | translatewiki repository ]].
Some projects, like MediaWiki and its extensions, use $1 and also [[ https://www.mediawiki.org/w/index.php?title=Manual:Messages_API#Notes_about_gender.2C_grammar.2C_plural | {{GENDER}}, {{PLURAL}} ]], $letters (for tvar), etc.
If it's possible to put $1 in one class and the other insertables in other classes and then use them all, it would be nice, but not a requirement for this task. It's enough to get all the projects that need //only// $1 to use the same class.
@Aklapper, this can be done in #google-code-in-2016.