Digging more into this problem I managed to find an easy solution. In the gtk+ install, there is a folder gtk+\share\locale which looked very much like internationalization files. I simply copied these files in strawberry_perl\perl\site\share and that made the trick.

Due to my limited understanding of how gtk and perl interact, I first thought that either perl was able to access the internationalization files in the gtk folder or that the installation had made the job for me.

Bottom line is: yes the internationalization set is set automatically based on computer settings, provided that the files are present in the perl install folder. If not, it defaults back to English without any warning of any sort.