Mutual intelligibility is defined by Wikipedia as "a relationship between languages in which speakers of different but related languages can readily understand each other without intentional study or extraordinary effort." There are levels of mutual intelligibility ranging from zero to 100%. The only language with which English enjoys a relatively high level of mutual intelligibility is Lowland Scots, which is considered to be a separate language by some linguists and merely a dialect of English by others. This is not to be confused with Scottish Gaelic, a Celtic language which is apparently fairly mutually intelligible with Irish Gaelic. There is extremely low mutual intelligibility between English and Scottish Gaelic.
Consequently, The Lowland Scots sentence "D'ye see yon hoose ower yonder" (Do you see that house off in the distance) would be relatively intelligible to an English speaker with no previous exposure to Lowland Scots.
But the Scottish Gaelic sentence "Dè an t-ainm a tha ort?" (What is your name?) would probably mean absolutely nothing to an English speaker with no previous exposure to Scottish Gaelic.
Some language sets enjoy a very high degree of mutual intelligibility. Norwegian and Swedish are, to a large degree, mutually intelligible, as are Bosnian, Croatian, and Serbian. Conversely, sometimes different dialects of a particular language will lack a significant degree of mutual intelligibility.
In Mutual Intelligibility in the Romance Languages, Robert Lindsay writes:
What is interesting is that everyone accepts that Spanish, Portuguese and Italian are separate languages, despite 54% intelligibility for Spanish and Portuguese and even higher for Spanish and Italian.Mutual intelligibility can vary in degree with respect to the written and spoken varieties of languages. In many cases, written forms are more mutually intelligible than spoken forms as speakers of one language can recognize similar features more readily in spellings than in pronunciations. A simple example is English and Afrikaans. "My pen is in my hand." could be a sentence in English or Afrikaans, with an identical meaning and read correctly by monolingual speakers of both. However, the words are pronounced differently, which could inhibit comprehension. Furthermore, mutual intelligibility may not be equal on both sides. It is apparently easier for a Dutch speaker to understand Afrikaans than vice versa and statistically easier for a Portuguese speaker to understand Spanish than the other way around.
However, in the cases of Austrian/Bavarian, Swabian (spoken around Stuttgart) and Mainfränkisch(Moselle Franconian, close to Luxembourgeois), these three languages are only 40% intelligible with Standard German. Their status as separate languages has infuriated lots of folks who just consider them to be dialects of German, or "cheap slangs" of some type or other. Yet they have a better case for being separate languages than Spanish, Portuguese and Italian do.
This brings up the point of the disparity between mutual intelligibility and lexical similarity (similarity of related words between languages). Differences in pronunciation and in the related forms themselves are largely responsible for this. An example is the Spanish word "tiempo," which is lexically similar to the French word "temps." Both mean "time" or "weather." Yet the pronunciations are so different that this could easily prevent comprehension. Lindsay writes:
We also learn, here, that no one can understand French except the French. Spanish, Portuguese, Italians, Romanians, no one can understand the damned French. This makes sense to me. I can’t understand a word of the local French-speaking tourists, and I had a semester of French. The always talk like they are holding their noses. This is interesting in light of the fact that Italian, Spanish, Portuguese and Romanian have 89%, 75%, 75%, and 75% lexical similarity with French. But all those similar words aren’t worth a hill of beans when it comes to understanding a Frenchman." [no offense, of course, is meant towards the French!]Mutual intelligibility may also decrease over time. English is very closely related to Frisian, spoken in Friesland in the Netherlands, and at one point, Old English and Old Frisian were thought to be mutually intelligible. Over the centuries, English and Frisian maintained some level of mutual intelligibility, giving rise to the sentence "Butter, bread, and green cheese is good English and good Fries," which is pronounced roughly the same and has the same meaning in both languages (in Frisian, it is "Bûter, brea, en griene tsiis is goed Ingelsk en goed Frysk"). Yet the two languages have drifted apart, with English upholding its 1000-year-old Norman French influence, and Frisian being influenced heavily by Dutch, to the extent that monolingual Frisian and English speakers probably could not understand each other today.
Lindsay addresses the subject: "Frisian and English have 61% lexical similarity, but in the Frisian video (featured in a prior post)...I could not make out a single word in five minutes. It appears that 60% lexical similarity and $1.89 will get you a Slurpee at a 7-11, but little in the way of understanding another language."
There is another pitfall with lexical similarity: false cognates. "Ano" exists in both Portuguese and Spanish, but while it means "year" in Portuguese, it means "anus" in Spanish. Similarly, a Spanish-speaking woman may be "embarazada," and an English-speaking woman may be "embarrassed." However, "embarazada" does not mean "embarrassed," but "pregnant."
Curiously, as some dialects of languages are more mutually intelligible than others (for example, some dialects of German and Dutch are more mutually intelligible than Standard German and Standard Dutch), it is thought that, by tracing a chain of mutually intelligible dialects, a "dialect continuum" may be established. In Europe, for example, Continental West Germanic, North Germanic, North Slavic, South Slavic, and Romance dialect continua are said to exist (see Wikipedia about "dialect continuum").
(Posted by language fan and friend Silas McCracken.)