There is no clear definition of what it means to "speak a language". A tourist who can handle a simple conversation with a waiter may be completely lost when it comes to discussing current affairs or even using multiple tenses. A diplomat or businessman who can handle complicated negotiations in a foreign language may not be able to write a simple letter correctly. A four-year-old French child would usually be said to "speak French fluently", but it is possible that he cannot handle the grammar as well as even some mediocre foreign students of the language do and may have a very limited vocabulary despite possibly having perfect pronunciation.

In addition there is no clear definition of what "one language" means. For instance the Scandinavian languages are so similar that many of the native speakers understand all of them without much trouble. This means that a speaker of Danish, Norwegian or Swedish can easily get his count up to 3 languages. On the other hand, the differences between variants of Chinese, like Cantonese and Mandarin, are so big that intensive studies are needed for a speaker of one of them to learn and even to understand a different one correctly. A person who has learned to speak five Chinese "dialects" perfectly is quite accomplished, but his "count" would still be only one "language", and some would argue that the differences between these dialects are greater than the differences between many European Romance languages.

As another example, a person who has learned five different languages such as French, Spanish, Catalan, Italian and Portuguese, all belonging to the closely related group of Romance languages, has accomplished something less difficult than a person who has learnt Hebrew, Standard Mandarin, Finnish, Navajo and Welsh, of which none is remotely related to another.

Furthermore, what is considered a language can change, often for purely political purposes, such as when Serbo-Croatian was assembled from Serbian and Croatian and later split after Yugoslavia broke up, or when Ukrainian was dismissed as a Russian dialect by the Russian tsars to discourage national feelings.

Bilingual model
Definition of "language"
Multilingualism in computing
Multilingual individuals