[danmaz74]

IIRC, studies show that if you learn a second language as an adult, learning a third one becomes easier (even if it is a completely different one) because you learned to learn a language. So yes, of course it is much easier to learn a similar language, but there is also an effect with completely different ones.

[PeterWhittaker]

I used to think along mcdaid's lines, that because I speak English and French natively German and Spanish were no problem, and because I'd learned Spanish, Italian was easy.

Then I learned Turkish, and later Arabic. Now I'm with danmaz74: Learning any language makes learning any other language easier.

Sure, knowing French made Spanish more accessible, but there is large gap between all of the languages I knew then and Turkish, and I picked it up pretty quickly. Same with Arabic some time later.

I attribute this to the mental "faculty" or "faculties" involved being flexible and responsive due to frequent use, rather than to the degree of similarity between the languages.

On the flip side, "use it or lose it" is definitely also true. I'm back to being native in two languages with a pretty decent third, because I don't use the other 4-5 at all.

They'd each come back pretty quickly with suitable immersion.

[maaku]

Which has a lot more to do with learning how to learn another language.