2.2.15 [RFC5545] Section 3.2.10, Language

V0022:

The specification describes the optional LANGUAGE parameter.

Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019  

Outlook only exports a LANGUAGE parameter for the SUMMARY property.

On import, Outlook uses the value of the first LANGUAGE parameter found in a SUMMARY, LOCATION, or DESCRIPTION property within a VEVENT component. All other instances of the LANGUAGE parameter are ignored.