@LilyNath Although there are English-only versions of Sims 4, I've never heard of a French-only version, and I doubt it even exists. People in French-speaking regions typically receive the global version, with all 18 languages available. It shouldn't matter though: what counts is the base game language, and once that's determined, Origin is supposed to allow the registering and downloading of packs bought anywhere.
However, it seems like a moot point—the product you linked me is listed as sold out. Is there a reason you don't want to buy directly from Origin? Sims 4 packs are currently on sale, and Get Famous is 50% off, at least in the U.S. So even with the price difference, I don't think you'd pay more.
To answer your other question, if you buy a physical copy of a pack, yes, you can register the product key in Origin, and that pack will be added to your game library just as if you'd bought it directly from Origin. But please be careful about where you buy, because some storefronts have been known to sell bad codes. Here's a list of authorized vendors of EA games:
https://help.ea.com/en-us/help/faq/where-to-buy-ea-games-v2/
Amazon itself is on the list, but the third-party sellers in Marketplace aren't covered.
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I don't work for EA. I'm just trying to help fellow players with their games.
