English sprouted in an island. Nobody outside that island understands them and vice versa (unless he or she goes through an adequate learning process, of course). English, Icelandic, Japanese, Welsh, etc., belong in this category.
The island languages of the United Kingdom go one step further; they are not mutually intelligible even among themselves. You have to be a native to the language to speak Irish or Welsh, tongues extremely difficult even for the Irish and the Welsh. In continental Europe, except for Basque or Hungarian, one language is usually related to the other.
Portuguese, Spanish and Italian, for instance, are mutually intelligible. This means that if you speak Spanish, you don’t need to know Italian to enjoy a simple conversation with an Italian.Nobody understands an English-speaking person when asking “What is this?” unless he knows English. However if a Spaniard who says “¿Qué cosa es esto?” will be understood by an Italian or by a Portuguese because the phrase is similar in those two languages, i.e. “Chè cosa è questo?” or “Que coisa é isto?”
For a Spaniard to communicate with a Frenchman will take longer, two or three weeks, maybe, but he’ll get there in two months without giving up his language. (I saw a friend from Brooklyn having a harder time trying to communicate with a man from South Carolina.)
Instant communication also occurs between German and Dutch, and German and its dialectic variations, which are much tougher than North Carolina’s dialectic variation in the States. Scandinavian languages are to German, what French is to Spanish, Portuguese or Italian.
Another group of mutually intelligible languages is the Slavic linguistic family. One can compare Polish, Bulgarian and Russian to Spanish, Italian and Portuguese, and Czech as the equivalent of French, in the previous example.
Lamentably, if you speak only English, you speak English and understand English and nothing else. Consequently, every other language is a foreign language, and this leads us to another misunderstanding.