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The LCPDFR settings (ini) file can be found in your GTA4 folder > LCPDFR > LCPDFR.ini.
The language setting does not exist in the ini by default, but should be added like this:
[Main]
AdditionalCopCarModels=
SuspectTransporterModels=POLICE;POLICE2;PSTOCKADE;
VehicleTrunkWeapons=Shotgun_Baretta;
AllTextMode=False
AmbientScenariosMaximum=2
AmbientScenariosMultiplier=250
DepartmentName=Liberty City Police Department
DepartmentShortName=LCPD
FirstStart=True
UsingPoliceStationMod=False
AutoSaveEnabled=True
Language=yourLangCode
Note the Language= property at the bottom.