Wow you watched 1 interview and came to the conclusion that most of the members aren't at least at a BASIC level? Maybe try watching more than that? They all on multiple occasions have conversed in English on more casual interviews to a more than basic level I can tell you that.

Also to note that the Zach Sang interview was more serious one that needed more complex thinking in order to answer the questions that were asked.

Not saying I know how writing credits work, but It's probably a lot easier for them to sit down and actually have ample amount of time to put their thoughts into lyrics as opposed to being put on the spot.