Is there a term to describe how two people can have a conversation in two different languages with each other.
My mom and I describe a 3 way conversation between a German tourist and my Mom, my Mom and myself, and a Parisian gendarme and myself as a tag team conversation. <shrug> Not the most elegant of conversations fraught with incipient charades and rummaging through 2 guidebooks for maps, but the German guy got his directions and we all had fun. [I speak French and English, my mom speaks English and German, the gendarme spoke only French and the German guy only spoke German.]
Here is a video (if this link works, fingers crossed) of an interview with a couple of players from the Norwegian soccer club Rosenborg. The reporter speaks Norwegian, the first player being interviewed speaks Danish, and the second one Swedish. They understand each other just fine, and the viewers are obviously assumed to as well.
(Nothing special about the video, it’s just the first one I thought of as an example of this sort of thing.)
Out of interest, for you non-Scandinavian speakers: Do the languages sound at all different to you, or all the same?
I would say of all the non-native speakers of English whom I have encountered, the Dutch have mastered it more than others in general.
Going back to the OP (without having read every post in this thread, I admit), isn’t one of the reasons the closeness of the Dutch and Nordic languages to English? As I understand it the closest language to English is Frisian, which is also closely related to Dutch.
And of course a lot of Danish and Norwegian would have been injected into English in the 600-odd years between the departure of the Romans and the Norman invasion (who of course were also Nordic, but speaking French by that time).
Happens in Spanish, French, Italian… as well, including among people who don’t speak English. There’s ads for ESL lessons based on “you know more English than you think”; the one I remember right now was two women talking about a trip one of them had taken and using help desk, boarding pass, overbooking… all of them badly pronounced because the speaker can’t speak English. It’s overdone in the ads, but real. There are also a lot of words which just get borrowed instead of attempting to translate.
Danes sound like they have a mouth full of mashed potatoes, Swedish is sort of monotone and Norwegians sing.
And I can say rød grød med fløde 
Danes sound like they have a mouth full of mashed potatoes, Swedish is sort of monotone and Norwegians sing. I know people say the reverse, but maybe it is the people I know, the guy from Copenhagen tends to be pretty laid back and Tomas from Oslo is pretty upbeat and tends to joke around.
And I can say rød grød med fløde
I can’t switch very fast between the two languages because I’ll then have trouble retrieving words right away. And it isn’t words like windowsill, or deoxyribonucleic, it’s words like salt, door. It happens regardless of which language I just switched to. It takes me a couple of seconds to readjust. I am not sure how common (uncommon?) it is. My husband and child are multilingual, but I haven’t noticed this (I am not sure anyone can tell about me either).
Yes. All three languages sound very different to me.
I don’t know if it’s been shown that ease of learning is strongly related to how closely related the languages are. It has not been my experience: as a native English speaker I have found Spanish much easier to learn than German, for example.
My suspicion is that when two languages are very close - so close that they are almost mutually intelligible without study - the relationship makes for ease of learning. But as soon as you go one step further, for example English-German, there is effectively no advantage. I doubt if a Finn would find Hungarian any easier to learn than I would.
Having said all that, Dutch may be close enough to English to confer an advantage, even if German is not. I find I can read Dutch reasonably well after relatively little study, en ik kan ook een betje spreken.
Of course there is an advantage. Try Russian or Japanese, and you’ll be completely lost compared to German.
I don’t think there is any real dividing line. The closer a language is to yours, the easier it is to learn it. There’s no line where it suddenly jumps from being hard to being easy.
That’s precisely the (commonly-held) belief I am questioning. Do you have any evidence that it is the case?
As I observed above, it is considerably easier for me to learn Spanish than German, despite the fact that my native language is Germanic.
Hey, after the first 10 years, learning Japanese is easy. Okay, maybe the first 20!
What is your native language? Most people find it much easier to learn a different sentence structure and syntax than a new vocabulary.
Hey, awesome! 
(It’s “beetje”, but hush, no nitpicking!)
There is: first there is a lot you recognise, that’s the easy part. Words jump out at you and you can make sense of simple sentences, because there are bits that resemble your native language and bits you remember from being made to read Beowulf at school. Then you start learning the grammar. German grammar is tough, and unless you really work at it and get lots of exposure you don’t really get a feel for the grammar. Spanish grammar is easier to get a feel for once you’ve studied it a bit, though you might not recognise as much at first when you come from English. You’d recognise more difficult words sooner than you would the very simple ones (recognise - reconocer!), because the more difficult English words are more often of Latin origins (in very general terms).
Dutch grammar is a little easier than German grammar and would be ever so slightly easier to follow for an English speaker, I think. Frisian even more so, supposedly.
Spanish is, in a certain way, closer to English than German is to English. Many of the more literate English words are Romance, and English borrowed much of its more literate vocabulary from French, while German stuck to Germanic terms much more for its literate vocabulary. But I’m just guessing now.
English
But in the case of German I needed to learn both. Neither the grammar nor the vocabulary was sufficiently close to English to be particularly helpful.
Reading this, I think we may actually be in agreement.
What I was disagreeing with was the idea that closeness of language relationships (so-called “genetic” relationships) was what counted in determining ease of learning.
If the point is that similarity to one’s own language is what counts, then I fully agree. With the caveat that languages can be fairly closely related and not very similar.
When I was in the netherlands in the '80s I was impressed by the number of english speakers. I asked a cab driver about it and he said that every public school taught dutch and english from grade one.
I only encountered one person who spoke no english.
Funny story, though; in our hotel they posted the evening meal in both dutch and english in the elevator.
One night there was no english translation, so we asked the desk clerk.He said it was “cow meat” with vegatables. In other words and “English boiled dinner.”
We went to the Greek place next door that night. 
We were ther in August but the temperature was in the 40s. I learned the word for gloves: handschoenen. Shoes for the hands.