At the top of any translatable page (example), there is a “Translate this page” small text which links to Special:Translate?group=page-PAGE_NAME&language=USELANG&action=page&filter= where PAGE_NAME is the name of the page, and USELANG is the user interface language code (example).
If MediaWiki interface is in French, language parameter is: &language=fr.
If MediaWiki interface is in English, language parameter is: &language=en.
This parameter is useless, because it automatically defaults to MediaWiki interface language when it is not set.
It is sometimes problematic, when page authors want to share this “translate” link: they share a link which will display a warning message: “The source language of this group is English. Please select another language to translate into.” because they have written the page in their own language (same than for their user interface).
Most of links shared to Translators-l have this issue.
Outcome
Removed the language attribute from the "Translate this page" link on the translatable page.