After play-testing half a dozen mods, I noticed that the all of the potions and arrows had slightly different grammar. I shrugged it off as a quirk of COBL, which I had just installed. "Arrows, Iron" and so on. Then I discovered that some of the names for potions were entirely french.

Things I have tried:
Uninstalling every single mod, one by one through BAIN
Copy-pasting a backup of my original Oblivion installation over the pseudo-french one.

Is there any way I can fix this short of reinstalling?

These are the only mods I had installed before the french invaded, so one of them is the culprit:

Redecoration 0.60
See you sleep 1.5.1
Dude where's my horse
All Natural 0.9.9
Lore Dialogue 300 1.3
Sittable rocks 0.54

Edit: Problem solved. None of the above are at fault.

Are you using the tweak names option in your bashed patch?

I tried enabling it (just the potions portion) as a fix, but it didn't have any effect.

Deactivating the bashed patch removes the french text. Unfortunately reinstalling Wrye Bash and rebuilding a new bashed patch causes the French to return. Not sure where to go from here. Where is information on potion names stored, and how is the bashed patch getting it?

What options do you have selected in the bashed patch? Anything under Import Names?

I had quite a few selected including import names, under which there are oblivion.esm, guard_names.csv and rational_names.csv. Hmm.... ***rebuilds patch yet again***

It will be rational names. Bunch of weird ones in there.

Yep, that was it. Thanks for the help!