The correct answer is actually not a single one of those which I listed.
Yes, it's kind of a trick question.
Imagine, going from knowing 0 language to 1, without any frame of reference.
This is what happens when you are learning your very first language... that is, your native language.
That's the hardest language to learn.
Learning a second language is easier, because it's just a matter of building vocabulary.
You already know what a tree is, for example. So learning a second word in a new language is easier.
And if you progress into learning a third and fourth language, as I have done, each new one gets easier still.
Yes, the more languages you know, the easier the next one is to learn. It doesn't matter which language it is.
Granted, some languages, like Chinese and Russian, have different symbols and "letters" for sounds, and that can take some adjustment, but the same thing can be said for these as well, once you learn a new symbol, it gets even easier to learn yet a third, or even a fourth.
English is my native language, as I am American.
As I live in California, and we have a lot of Mexicans, I chose to learn Spanish while in High School. People living in Northern States near Canada may choose French, as a lot of Canadians speak French.
When I was in X-ray school, I was seeing a Russian girl, so I learned a bit of Russian. I got quite good at reading the symbols, however, I did struggle with what I was actually saying. I didn't stay with her long enough to get too far.
Then I learned German. I listen to a lot of German Industrial music, and I wanted to know what the lyrics were. So I took 3 semesters. Then I got the chance to travel to Germany for a month.
But, one thing I probably should mention.
You probably already know I am in the medical field, and therefore must know a lot of medical terminology. That, in itself, is like learning a new language, because just about everything medical is still named in Latin or Greek.
But, the nice thing about English, especially American English, is that it incorporates so many other languages into itself. Deja-vu is French, but we say it in English. You see the prefix uber all over the place, that's a German prefix for over/above. The names of the planets and our sun are ancient Greek gods. (Is it Greek? or some other mythology? -- not really important... they aren't English.)
Has anyone ever noticed how I like to ramble? I'm pretty prolific when it comes to rambling, don't you think? heh