leading international company

Und international bzw. internationally leading company wäre somit undenkbar falsch oder geht eins davon trotzdem?

Ein kleiner Kommentar hierzu wäre noch nett.

an international leading company = ein internationales führendes Unternehmen
an internationally leading company = ein international führendes Unternehmen

I agree with noli; 'leading international' is the only idiomatic choice, even though it's not a literal translation. 'International company' is one concept, and 'leading' describes it all.

'Internationally leading' isn't exactly grammatically wrong, but we would not use it, because it suggests that the company is doing well overseas but poorly at home -- internationally it's leading, but domestically it's lagging behind.

Another option: Z Company is an international leader in (widget manufacturing).

Or 'a global leader in ...'

Sorry, das hätte ich wohl besser kommentieren sollen. Ich wollte keinen Übersetzungsvorschlag machen, sondern die Bedeutungsunterschiede in den beiden Varianten aufzeigen.

You need the adverb "internationally" here as it is used as a modifier to the ing-form "leading," which cannot be modified by an adjective, i.e. by "international."