Are you sure you can use "it's" in this context?
You can say "It's in French" instead of "It is in French."
You can say "I think it's in French" instead of "I think it is in French."
But "Most of it's in French" in lieu of "Most of it is in French" sounds odd. It's like saying "Most of it's good." Doesn't sound right.

So you would consider "Most of it's good" as acceptable, then?

I commented on this a few years ago. I still feel it is wrong to say "it's" in this situation.

This sounds OK to my American ear.

The following sounds natural to me, too.
Most of it's good.

That's on a lot of web pages, too.

https://www.google.com/search?q...2Most+of+it%27

This, to me, belongs in the same category as "your beautiful," "he must of pissed him off, "I could of killed him."

Can you find evidence of "it's" used this way being OK in British English?

Those 2 examples are spelling errors.

your beautiful => you're beautiful

I could of killed him. => I could've killed him.

"It's" is a contraction of "it is."

I know. But can you find evidence of this use in British English? It just sounds wrong to me.