At one time at the height of the Roman Empire Latin was the same language spoken in Rome(Italy),
Spain, Portugal and France with just a little dialectical variations like how Spanish, Portuguese
and French vary from country to country now. So if you spoke the Latin of any one of these countries
you could go to any one of the other countries and be understood.(By the way this Latin language was
spoken in Spain for 840 years before it became Spanish.)
Italian has 82% similarity with Spanish.
Spanish has 75% similarity with French.
Spanish has 89% similarity with Portuguese.
Italian and French have 89% similarity (yet it
seems easier for an Italian speaker to learn Spanish
and course visa versa.)
So if you know Spanish it is easier to learn Italian first
an then French.
If you're an Italian speaker and learn Spanish it will then be
easier to learn Portuguese. Your Portuguese will then help
you learn French and visa versa.
Let's not forget Romanian and Catalan which are also Latin based languages.
If you know Italian then you can learn Romanian and Catalan piu facile./easier.
If on the other hand you are just a poor little old English only speaker you'll still be ok
because 60+% of English vocabulary comes from Latin. So you're going to be just fine
no matter what. Relax, take a deep breath. Speed limit 5 miles per hour.
Then you as an English speaker who learns Italian will be able to learn Spanish and
then Portuguese and then French or visa versa.