Correct spelling


Correct spelling, explanation: internationally is an adverb made from the adjective international and the adverbial suffix -ly. The process here is very straightforward and so the l is doubled as it occurs in both the base word and the suffix.

Definition of internationally:
1. Concerning many countries
Lady Gaga is an internationally recognizable celebrity.
I traveled internationally for many years and I know a lot about different cultures now.

Incorrect spelling


Incorrect spelling, explanation: considering that the stem word in internationally is international and the adverb-making suffix is always -ly and not only -y, it’s just not possible for internationaly to be correct, as it would mean one of the l’s has been deleted which doesn’t happen in a language without a good reason.