This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Defining a language parameter other than “L”¶
The rule in the TYPO3 community is that L as a parameter in the URL
means language. If you are using another parameter, you can set the
languageGPVar property in TypoScript.