We can follow the below procedure if we want the mandatory language for the portal to be set to a single specific language.
1. Login to the Portal Server
2. Navigate to the following path in the SAP installation directory
\\usr\sap\SID\JID\j2ee\cluster\server0\apps\sap.com\irj\servlet_jsp\irj\root\WEB-INF\portal\system\properties
3. Open the file 'prtDefault.properties'
4. Deactivate the default language settings of the portal by adding '#' before the parameters as below.
#request.defaultlanguage=en
#request.defaultcountry=us
5. Add the following parameters to the file
request.mandatorylanguage=en
request.mandatorycountry=us
6. Save the file and restart the portal
The parameter 'request.mandatorylanguage=en' will override the browser language settings of the end users and will display the contents in 'en'(English) only.