What does the define slope limits function does and how do you determine the values?

https://www.rocscience.com/help/slide2/documentation/slide-model/slip-surfaces/slope-limits/define-slope-limits

Is the code open source? How do I access it? Can I contribute by adding a language and local features?

source is not available yet but you can download the localization files from download page (http://geowizard.org/download_hyrcan.html) and start translating the text. please make sure you follow the instruction provided in the zip file.

what language do you want to translate the text to?