Currently the prefix cmn is an interwiki prefix and not and interlanguage prefix in Wikipedia: https://en.wikipedia.org/wiki/Special:Interwiki.

The wikitext [[cmn:Foo]] makes an inline interwiki link to https://zh.wikipedia.org/wiki/Foo instead of interlanguage linking the article with https://zh.wikipedia.org/wiki/Foo.

cmn is the language code for Mandarin Chinese and part of the macrolanguage Chinese (zh). The parts of the macrolanguage Chinese (zh) are (https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry):

• cdo: https://cdo.wikipedia.org/
• cjy
• cmn: https://zh.wikipedia.org/ the alias https://cmn.wikipedia.org/ is missing
• cpx
• czh
• czo
• gan: https://gan.wikipedia.org/
• hak: https://hak.wikipedia.org/
• hsn
• lzh: https://zh-classical.wikipedia.org/ with alias https://lzh.wikipedia.org/
• mnp
• nan: https://zh-min-nan.wikipedia.org/ with alias https://nan.wikipedia.org/
• wuu: https://wuu.wikipedia.org/
• yue: https://zh-yue.wikipedia.org/ with alias https://yue.wikipedia.org/

The wikitext

[[cdo:Foo]]
[[cmn:Foo]]
[[gan:Foo]]
[[hak:Foo]]
[[lzh:Foo]]
[[nan:Foo]]
[[wuu:Foo]]
[[yue:Foo]]

makes interlanguage links for all languages except for cmn. cmn is only an interwiki prefix and makes an inline interwiki link. (https://en.wikipedia.org/wiki/Special:ExpandTemplates?title=Special%3AExpandTemplates&wpInput=[[cdo%3AFoo]]%0D%0A[[cmn%3AFoo]]%0D%0A[[gan%3AFoo]]%0D%0A[[hak%3AFoo]]%0D%0A[[lzh%3AFoo]]%0D%0A[[nan%3AFoo]]%0D%0A[[wuu%3AFoo]]%0D%0A[[yue%3AFoo]])

The prefix cmn should be an interlanguage prefix like the other parts of the macrolanguage Chinese (zh).