Which do you think is more important to learn a foreign language to read a lot, or to listen to a lot? then, is the sentence above correct?

The question should say "important in learning a foreign language" or "important when learning a foreign language." You can't say one is more important since both are absolutely necessary. However, understanding spoken language is always more difficult than reading since you can't go at your own pace, or go back to repeat something you didn't understand. So listening takes more effort and practice.

Both.  Written material doesn't 'disappear into the air' the way spoken language does.  You can read and reread a written source to learn vocabulary, how to use it, and good grammar.  To be fluent in a language, you have to speak it.

If you really want to be fluent, I would say both. You can also add: write a lot.

probably to listen a lot

Which do you think is more important: to learn a foreign language for the purpose of reading or for the purpose of listening/conversation?

For me, both [and more]. First listening, then speaking, then reading, and finally writing.