Nailing Jello to the wall: Language Identification
The Ethnologue deals with the languages of the world, so it would seem to be important that we be able to say what we mean when we refer to "a language." What is a language? Well, Webster has 14 different definitions of the noun, not counting two for "language" as a verb (I language, you language, he/she/it languages). Perhaps a good starting point would be the most basic idea of language as “a systematic means of communicating by the use of sounds or conventional symbols " [http://www.websters-online-dictionary.org/definition/language]. For the purposes of the Ethnologue we add some additional qualifications. For example, we don't include animal languages (bees dancing, etc.), computer languages, or humanly constructed languages that don't have any native speakers (e.g. Navi, Klingon), even though these systems do meet the criterion of being a "systematic means of communicating by the use of sounds or conventional symbols." Though some of these language-like systems have been assigned ISO 639-3 codes identifying them as distinct languages, Ethnologue doesn't report on them. You can see the complete list of languages identified by ISO 639-3 here.
When we look at language outside the dictionary in the real world, things do not get much easier. Most people tend to look at a language as a concrete whole, assuming well-defined boundaries, in spite of understanding that there are a number of varieties, dialects, jargons, and even idiolects associated with any given language. If it is our own language, taking English as an example, we figure that, if we can understand what the other person is saying, that person is speaking in English. But what if they believe they are speaking in English and we don’t understand them? This has happened to me more than once on travels within the United States!
Mutual intelligibility is indeed one way to determine where one language stops and another one starts. But, in my experience, I believe that the other person did understand me when I spoke; I just could not understand him when he replied. Intelligibility is not necessarily a two-way street. There are a number of factors that contribute to or inhibit intelligibility. So, then, what if neither person understands the other? Again, we run into problems when we’re speaking two different dialects from the extreme ends of a dialect chain: I speak Dialect A and can readily understand Dialects B, C, and D. However, Dialect E is like a foreign language to me but not to its nearest neighbors. So then, where do we “cut” the dialects into two or more languages? If, for example, we decide to make the cut between Dialect D and E, people from those two dialects can easily understand each other, and they won't accept that they are speaking two different languages.
A language is distinguished not only by intelligibility but also by ethnic, cultural and/or political identity. In some cases, one ethnic identity can include speakers of several languages or even people who no longer speak a particular language. For example, someone who self-identifies as an Albanian may be a speaker of one of four distinct Albanian languages. These four languages are, for the most part, not mutually intelligible. And an Albanian might also be one of those ethnic Albanians who has given up their mother tongue and now speaks another language altogether.
On the other hand, there are many languages that are mutually intelligible but nevertheless are considered to be separate. One example would be Hindi [hin] and Urdu [urd], where there are both cultural and political divides as well as different writing scripts in use, yet the two are largely mutually intelligible in spoken form. Another example is the case of Bulgarian [bul] and Macedonian [mkd], which are mutually intelligible varieties in the same dialect chain separated by national borders.
As you can see, defining what constitutes a language is not so easy after all, sort of like nailing Jello to the wall, as someone has said! Every language is in constant flux, especially in this age of international communication and the globe-shrinking internet. We discuss this whole topic in more detail in the Introductory material on this website found here.