As Guki42 found out here, automatic language switching is broken in InkscapePortable 92.1. The folder structure seems to have been altered in this version. Here is a fix for a new revision:
Change Line 23
CheckIfExists=%PAL:AppDir%\Inkscape\locale\%PAL:LanguageCustom%\*.*
to
CheckIfExists=%PAL:AppDir%\Inkscape\share\locale\%PAL:LanguageCustom%\*.*
This topic is a translation and for the bug tracker.