Thread:Project:Support desk/How to create a "constant"

With wikis with possibility of translation, as MediaWiki, and to facilitate the translation of articles, these wikis use "constants". For example: ...contribution under the [$cc0 CC0]. See Public Domain Help Pages for more info. transcluded to: ...contribution under the CC0. See for more info. How to create these constants or variables (here $cc0 and $pd-help)?