AP writes: A question: how is it decided that the cut-off between a language and dialect is 90% MI? Rather than 95% or 85%? Is there an agreed-upon standard?
There is no agreed-upon standard, as the whole dialect versus language and even the notions of mutual intelligibility can be fraught with much controversy in the linguistic community. However, there can be general agreement about a few things in these areas.
The difference of course is completely arbitrary, but above 90%, most speakers regard their comprehension as “full” or say things like “I understand it completely” and most people refer to the other lect as a dialect of a language as opposed to a separate tongue altogether.
Below 90%, it starts getting a lot more iffy, and down towards 80-85%, people start saying things like, “I understand most of it but not all!” and people start regarding the other tongue as possibly a separate language, although still tends to be quite a bit of controversy around whether the L2 is a separate language of a dialect of another language because the intelligiblity is so high.
Around 80% comprehension, it gets hard to talk about complex or technical things. Communication about such things is significantly impaired at this level. Also after studying Ethnologue for a very long time, I noticed that they tended to use 90% as a cutoff for language versus dialect most but not all of the time. This was most important in the huge section on Indian languages of Mexico, in which separate languages (part of a larger macrolanguage) are often spoken only a village away.
Some famous linguists who are acquaintances of mine (they have Wikipedia pages) told me that they thought that 90% was a good metric. Also I have a long article coming up as a chapter in a peer reviewed book being published out of Turkey. The main Turkologist I worked with on that chapter told me that he thought 90% was a good metric.