Question Roundcube Content-Language

[Marcs84]

Part of header mail create from Microsoft Outlook 16.0
....
Content-Type: multipart/alternative; boundary="----=_NextPart_000_074A_01D55F19.2A9B0040"
X-Mailer: Microsoft Outlook 16.0
Thread-Index: AdVfCGSK+zpBv7BFT6SkMXDvIKzgtQ==
Content-Language: it
Disposition-Notification-To: <[email protected]>


Part of header mail create from roundcube
...
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Date: Fri, 30 Aug 2019 19:56:22 +0200
From: [email protected]
To: [email protected]
Subject: test
Message-ID: <[email protected]>
X-Sender: [email protected]
User-Agent: Roundcube Webmail/1.3.8

The content-language is not present.
How can I add content-language to emails sent with roundcube?


Thank you
Marco