Hi,
it's the OLAP connection defined in BI4 that will be the determining factor here, plus the STS (security token service) which handles BW SSO to BICS.
i would have thought, either the connection is hardcoded to EN (or another language code), else you need to leave it open (unticked_ so that the SAP User's PVL (preferred viewing language) is picked-up at runtime
Regards,
H