Ok, I found my solution in the documentation:
New extended syntax for more flexibility. Use for example {gallery}mygallery:200:140:0:2{/gallery} to instruct the plugin to fetch the images inside the "mygallery" folder, resize them to 200px width and 140px height, display the gallery in normal (grid) mode, create and use a en-GB_labels.txt file inside the "mygallery" folder, ready for you to edit.
First 2 numbers are width and high of the thumbnails.
Number 3 is "0" for the normal (grid) mode or "1" for a single thumnail.
Number 4 is the selected language label.txt file (0=none; 1=default (in my case de-DE); 2=en-EN ...)