The European Union works in 24 official languages, and Regulation No 1 is the instrument that lists them

Background

The EU has 24 official languages: Bulgarian, Croatian, Czech, Danish, Dutch, English, Estonian, Finnish, French, German, Greek, Hungarian, Irish, Italian, Latvian, Lithuanian, Maltese, Polish, Portuguese, Romanian, Slovak, Slovenian, Spanish and Swedish.
Every time new members have joined the EU, they have added to the number of official languages.
The Council establishes the rules on the use of languages by the EU institutions, acting unanimously by means of regulations adopted in accordance with Article 342 of the Treaty on the Functioning of the European Union.
Those rules are laid down in Regulation No 1, which states that the institutions have 24 official and working languages.
Multilingualism is enshrined in the Charter of Fundamental Rights: EU nationals have the right to use any of the 24 official languages to communicate with the EU institutions, and the institutions must reply in the same language.
Legal acts and their summaries are available in all official EU languages.
Meetings of the European Council and the Council of the European Union are interpreted into all official languages, and Members of the European Parliament have the right to use any official language when speaking in Parliament.
English remains an official EU language, despite the United Kingdom having left the EU, as long as it is listed in Regulation No 1.