Is there a way to make your own custom translation and translate every thing in the in-game interface from english to your choice of words/language?

Most of the game's interface is in UIText.iff, which is found in your game's The Sims\GameData folder. Back it up first, by making a copy of it and putting it in a safe place, then open it with IFF Pencil2 or something similar, and change the text to the text of your choice.

Changing the text of individual objects in the catalog would be a lot harder, since there are many, many objects in the catalog. But just changing the user interface text should be a snap.

You can get IFF Pencil 2 at Sim Depot's Must-Have Programs page.

I was in a project to translate the game into Russian, so I can help. Basically the entire translation is in the .far archives. Well, as they said above in files like Live, UiText and so on.

Seen your DM. Dunno why, but MTS seems broken as I cant pm you. Share your discord or whatever in there so I can add you.

Try this link (It leads to a chat with me): https://t.me/+csmZ1YGHhjY2NjFk

(I hope strangers don't come in)