Aside from that, don't underestimate the amount of thinking we do without language. What do we call that - raw braincode? So in that sense, we're all bilingual, even those only speaking English. I.e. you can think "I'm going down to the post office to post a letter with a stamp on it" without language.
True. But for single-language speakers (and even some bilinguals who haven't thought about this), it is easier to conflate the raw thinking and the language they associate with it (which we know happens after the fact)... because they only have one language to work with. A bilingual on the other hand will understand the concept instantly if they were to think about something as themselves in one language and the same thing as themselves in another language... and they don't have to rely on poorly designed studies to know these things.
Also, depending on how integrated the different language personalities are (probably depends on whether they learnt the language as a child or as an adult), they should also easily be able to see that they are not one personality but at least two personalities embodied in one being (which can then be taken further into understanding that what we think of our "self" is made up of many different personalities based on different contexts... which is all acquired from upbringing).
