Many languages in the EU are related, that is they evolved from the same proto-language. Related languages have some amount of similarity, for example, sharing basic vocabulary or grammar rules. These related languages belong to the same language family. Each family contains several branches. It is relatively easy to learn a second language within the same branch. For example, a speaker of English finds it easier to learn Danish than Spanish.
More than 90% of EU citizens speak a language that belongs to a branch of the Indo-European language family:
The Greek, Albanian, Baltic, and Celtic branches also belong to the Indo-European family.
The Uralic languages are the second largest language family in the EU, containing (among others) the Finno-Ugric languages (e.g. Finnish, Hungarian, and Estonian).
There are also many minority languages, local and so-called emigrant languages from many language families that are used in the EU. Users of sign language are in some sense also a minority and raise special requirements relating to language usage and translation rights.
In the last century, several planned languages have acquired a speaking community. In contrast to the restricted artificial languages for computer languages (C++, Perl, Java, etc), international planned languages, such as Esperanto are full-fledged human languages, often deeply connected to the EU countries.
Latin, once the unified language of Europe, also has religious and cultural significance in the EU. As the language used in Roman-Catholic masses and the official language of the Vatican, Latin remains important to the scientific community as the source language for much of scientific terminology.