First question: Why?
What language do you want to add as there are already several languages translated by the base game (en, fr, de, jp) + patches/addons for other languages (es, pt, ru, pl, it,...).
Check the other language addons in oder to see how they translated everything:
https://www.esoui.com/downloads/info...WIP-74.26.html
https://www.esoui.com/downloads/info1742-EsoPL.html
https://www.esoui.com/downloads/info...lationWIP.html
https://www.esoui.com/downloads/info...2018ESOBR.html
The ingame localized texts of ESO can be "partially" found here:
ESOUI source codes:
https://github.com/esoui/esoui/tree/master/esoui
ingame localized texts:
https://github.com/esoui/esoui/tree/...melocalization
ingame internally localized texts:
https://github.com/esoui/esoui/tree/...melocalization
All other data like quest names, skill names etc. are kept elsewhere and must be translated by other means, check the other language addos how they do it then.