There is a distinction between the newsletter’s language and the language of a recipient segment (subscribers to a mailing list). There is no link between these two plugins.

The items displayed in the sign-up form enabled by are mailing lists, not newsletter subscriptions.

If a mailing list has no language segments, it appears in its original language across all languages ​​enabled on the site.
If the mailing list supports multiple languages, you must apply multilingual tags to its public title and tooltip.

<multi>[fr]rubrique CGAS [de]Abschnitt CGAS [it]sezione CGAS [en]section CGAS </multi></

To summarize :

• you subscribe to one or more mailing lists (ML)
• you can belong to a language segment of a mailing list (ML)
• each mailing list can serve as the recipient for a newsletter (NL)
• sending a newsletter (NL) can be restricted to a segment of a mailing list (ML)
• and a newsletter (NL) can have a language