Custom civs can't be given descriptions without overriding the base game

I’ve been trying to make four custom civs for the past few days, and I have succeeded at every point except one: the tech tree description.

If I use the existing Britons strings and simply paste the modified ones in the mod-strings file, it modifies it for both the mod and the default dataset, despite the default dataset not being touched by my changes.

If I try to use a new string, as I have for the UUs and UTs, it simply refuses to recognize it. It will change the name but not the description, instead defaulting to the Britons default description. I have even changed the strings to match with the new civ ID, but it won’t accept it.

EDIT: I fixed it. It turns out that the internet is wrong, having two mods is incorrect. I moved the language files into the .dat and now it works. I’m going to redo all of the other things over the top of the Britons too, to save space.