Oh yes i saw that, and i tried to copy arrays in relatives data in my save game, but as i said welp. Not even close xD. So what should code look for adding more traits than one?
Well first off, stop calling them traits! They're Specializations. The game has traits and they mean something else entirely.
To make it support this, you'd have to find every reference of .spec and see if/how it should be updated. Exactly how it changes depends. Yould for one need to make newly created characters define this as an array, and for loading to convert it from an array. Or you'd need to make the code handle if it was an array or just a plain string. Then maybe a helper function to check for a given spec. Then every place that checks for a spec would need to be update to use said function.
If you're asking this question, then I don't think you're up for making the necessary changes. You're looking at changing somewhere around 92 places in code and at least several of those are non-trivial changes.
You don't need to modify anything for traits, it's already an array in the file. Just add whatever traits you want, only need to make sure you type it exactly as the game wants, as i believe it's case sensitive on it. For example, here's the traits for one of my slaves:
"traits": ["Submissive", "Infertile", "Pervert", "Masochist", "Likes it rough", "Bisexual"],