Learning a language through another language

How easy is it to learn a foreign language using a foreign language that you have already learnt?

Also, I have noticed that some foreigners on this forum are so proficient in English, they can use it freely and precisely, talking about other languages from an English point of view.

I'm learning Hungarian and modern Greek through English

<<How easy is it to learn a foreign language using a foreign language that you have already learnt?>>

English is not my first language and it's the only language I've been learning, but I think that if I ever learn another one, I'll definitely learn it through English. The reason is very simple: there's much much more material available in English for learning languages compared to what you can find in my native language.

But if you learn language X through language Y, language Y must not be a limitation, so you need to know language Y very well.

I'm learning French through Spanish