they didn't say "the original in French" but "the original French"

Right. You're not a native English speaker, I guess, so I'll offer some advice on how you could have read this correctly.

You can tell that "the original French" is a noun phrase, because it's the object of the preposition "in", and prepositional objects are always noun phrases in English. Given that, there are two possible parses: either "original" is an adjective and "French" is a noun, or "original" is a noun and "French" is an adjective. In English, adjectives nearly always precede nouns, so the second possibility is very weak.

The noun "French" in English can refer either to the French language or (always with "the") the French people. But interpreting the phrase as meaning "in the original French people" is clearly incorrect, because a museum is too large to fit inside a person, and even a somehow miniaturized museum could only be inserted into one person, not many people.

So the phrase unambiguously means "in the original French language", that is, "in the French-language text originally written".

I hope this helps!

"In the original French" means the original French language in which Tintin was written.

"In the original French" does not refer to the nationality of the author or the location of the museum.