This issue was introduced with the new IIS web adaptor in 11.1 and has been addressed. This fix has been included in the ArcGIS Web Adaptor (IIS) 11.1 Reliability Patch which is available for download.
https://support.esri.com/en-us/patches-updates/2023/arcgis-web-adaptor-iis-11-1-reliability-patch
With the patch installed, a generateToken request with the 'Content-Language' header works as expected.