The Difference Between a Language and a Dialect

In my opinion, a dialect should have 90% intelligibility or more. There are others who put it at 50%, but I think that is ridiculous. Everything above 50% goes to dialect and everything below 50% goes to language. Ethnologue uses 90% (in general).
For instance, Moldavian is widely regarded as a dialect of Romanian and the Moldavian linguistic nationalist movement has bit the dust. That is because there is 95-100% intelligibility between Romanian and Moldavian. Initially, Ethnologue granted Moldavian a language code, but then they revoked it later, probably because Moldovan is not a real language, plus the issue was really blown out of proportion by Romanian nationalists.
The Valencian linguistic nationalist movement has also bit the dust, as Valencian is 90-95% intelligible with Catalan. In contrast, Galician is only 85% intelligible with Portuguese, hence is regarded as a separate language from Portuguese, though Portuguese nationalists regularly get very upset when you say that.
Splitting gets very difficult in places like Mexico where in some mountains, just about every village has its own lect. In some cases, Lect A has 100% comprehension of Lect B, but Lect B only has 85% comprehension of Lect A! This lack of one to one correspondence is also a very common and strange feature of world lects.
Since each language needs over a 90% comprehension of the other to be considered dialects, Lect A and Lect B are split into 2 separate languages, even though Lect A speakers understand 100% of what Lect B speakers say!
For instance, Teotitlán del Valle Zapotec has 100% intelligibility of San Juan Guelavía Zapotec, but San Juan Guelavía Zapotec has only 59% of Teotitlán del Valle Zapotec, therefore Teotitlán del Valle Zapotec needs to be split to a separate language, but I guess Ethnologue has not gotten around to it yet.
You may ask why I am using the term lect. The confusion about languages and dialects is so great that a lot of times we just use the word “lect” as in “tongue” to refer to something that may be either a language or a dialect, or one or the other. In this way, you can also lump languages and dialects together in one term, which is handy.
The notion of what is a language is what is a dialect goes on and on. A famous maxim puts it that a language is a dialect with an army and a navy.
Bosnian, Croatian, and Serbian are examples of dialects that have recently obtained the status of languages. Since they were involved in wars where they were killing each other, the Hell if they wanted to say they spoke each other’s languages. Due to sociopolitical issues, Ethnologue has accepted these new languages, but Montenegrin has been rejected up until now. A newly free Montenegro is now pushing for status as a separate language.
Macedonian is said to be nothing more than a dialect of Bulgarian, but since it is now an independent country, it has been granted the status of a language.
The differences between Belorussian and Russian and Ukrainian and Russian are said to be not so great. Further, Danish, Swedish and Norwegian speakers are said to be able to understand each other. Breton and Welsh are said to be somewhat intelligible.
Czech and Slovak speakers can generally understand each other, but here we have a dialect chain, where the speakers at opposite ends of the chain are unintelligible. For instance, Czech speakers cannot understand Slovak speakers of the far eastern dialects, and Slovak speakers cannot understand Czech speakers of the far western dialects. Dialect clusters and chains are very common across the world and make it a real headache when it comes to splitting up languages and dialects.