There are some behaviors that are nearly universal amongst the bilingual, at least in English* Many of these tendencies are annoying but they appear to be organic, and will probably be a part of the learning process.
This question occurs with bizarre predictability, in exactly the same form and always in English.
Nobody understands anything completely, so the question is problematic already. And why in English?
It's mysterious behavior, and ubiquitous. There's one in every crowd.
Almost as annoying and more prevalent is the eagerness of the bilingual to help, interrupting a conversation to offer translation.
If two people are trying to learn each others' language and are having minor difficulty, that's how people learn. A brief delay to allow people to learn would be courteous. But that's not how it works. You'll get help that is no help at all. It never stops. To convince even one person, you may have to beg. It probably won't work. And there are always more of them.
The bilingual, when they bother at all, will probably do so in a patronizing manner.
Sometimes, only trying to help, they will talk so slowly that it's hard to even pay attention. The rhythm is unnatural and the practice is irritating, which adds another element of distraction.
Sometimes the bilingual will begin speaking in his language and it may seem like he's giving you a chance.
Usually, he's just trying to "lose you" to push you beyond your ability. You want to speak in our language? Here it is more than you wanted. He's trying subconsciously, not intentionally to dispense with your efforts, and get back to speaking English.
If you can display an unusual ability with a language that most people don't bother to learn, there are going to be people who will want to show your skill to their friends. This is flattering. But now, with the pressure to be impressive, you're supposed to know how to respond. And again here, the rhythm of conversation is often unnatural, not conversational in a normal sense. It's awkward.
The bilingual are an enormous pain in the ass, and nearly useless to a student of their language.
*These behaviors are probably a part of all encounters where comprehension is non-symmetrical.
English may be a special case, though, since it is the first global tongue.