The official and universal language used in Spain is Spanish (español) which is a member of the Romance family of languages (others include Portuguese, Catalan, Italian, French, and Romanian). Many people, especially outside Castille, prefer to call it Castilian(castellano).
However there are a number of languages (Catalan, Basque, Galician, Asturian, etc.) spoken in various parts of Spain. Some of these languages are dominant in their respective regions, and, following their legalisation in the 1978 constitution and they are co-official with Castilian in their respective areas. Of these, Catalan, Basque and Galician are recognised as official languages according to the Spanish constitution. Apart from Basque (whose origins are still debated), the languages of the Iberian Peninsula are part of the Romance family and are fairly easy to pick up if you know Castilian well. While locals in those areas can also speak Spanish, learning a few words in the local languages where you are traveling will help endear you to the locals.
- Catalan (Catalan: català, Castilian: catalán), a distinct language similar to Castilian but more closely related to the Oc branch of the Romance Languages and considered by many to be part of a dialect continuum spanning across Spain, France, and Italy and including the other langues d'oc such as Provençal, Beàrnais, Limousin, Auvernhat and Niçard. Various dialects are spoken in the northeastern region of Catalonia, the Balearic Islands, and Valencia (where it is often referred to as Valencià), east of Aragon, as well as neighbouring Andorra and southern France. To a casual listener, Catalan superficially appears to be a cross of Castilian and French, and though it does share features of both, it is an independent language in its own right.
- Galician (Galician: galego, Castilian: gallego), very closely related to Portuguese, Galician is spoken in Galicia and the western portions of Asturias and León. Galician predates Portuguese and is deemed one of the four main dialects of the Galician-Portuguese language family group which includes Brazilian, Southern Portuguese, Central Portuguese, and Galician. While the Portuguese consider it a dialect of Portuguese, Galicians themselves consider their language independent.
- Basque (Basque: euskara, Castilian: vasco), a language unrelated to Castilian (or any other known language in the world), is spoken in the three provinces of the Basque Country, on the two adjacent provinces on the French side of the Spain-French border, and in Navarre. Basque is unrelated to any Romance language or to any branch of the Indo-European or Indo-Iranian family of languages. It currently remains unclassified and is deemed a linguistic isolate, seemingly unrelated to any branch of the linguistic family tree. How it got to where it is now and whether it might be (however distantly) related to any language, living or dead is one of the most hotly debated issues in linguistics and rivers of ink have been and will probably be spilled over the issue for the foreseeable future.
- Asturiano (Asturiano: asturianu, Castilian: asturiano, also known as bable), spoken in the province of Asturias, where it enjoys semi-official protection. It was also spoken in rural parts of León, Zamora, Salamanca, in a few villages in Portugal (where it is called Mirandes) and in villages in the extreme north of Extremadura. While the constitution of Spain explicitly protects Basque, Balearic-Catalan-Valencian under the term Catalan, Galician, and Castilian, it does not explicitly protect Asturian. Still, the province of Asturias explicitly protects it, and Spain implicitly protects it by not objecting before the Supreme Court.
- Aragonese (Aragonese: aragonés, Castilian: aragonés, also known colloquially as fabla), spoken in the north of Aragon, and is not officially recognised. This language is close to Catalan (specially in Benasque) and to Castilian, with some Basque and Occitan (southern France) influences. Nowadays, only a few villages near the Pyrenees use the language vigorously, while most people mix it with Castilian in their daily speech.
- Aranese (Castilian: Aranés, Catalan/Aranese Occitan: Aranès), spoken in the Aran Valley and recognised as an official language of Catalonia (not of Spain), alongside Catalan and Castilian. This language is a variety of Gascon Occitan, and as such is very closely related to Provençal, Limousin, Languedoc, and Catalan.
In addition to the native languages, English and French are commonly studied in school. English is in general the more widely spoken of the two, though proficiency among the general population tends to be poor.
That being said, most people in Spain's important tourist industry usually have staff members who speak a good level of English, and particularly in popular beach resorts such as those in the Costa del Sol, you will find people who are fluent in several languages. English is also generally more widely spoken in Barcelona than in the rest of the country. As Portuguese and Italian are closely related to Spanish, if you speak either of these languages, locals would be able to puzzle you out with some difficulty. In some areas frequented by German tourists, such as Mallorca, you will have a good chance of being able to communicate in German. In addition to that, there has been migration from Spain to Germany since the first guest workers arrived in the 1950s and today many young people move from Spain to Germany to study or work. Many of those migrants have since returned to Spain and there are also a number of German retirees.
Castilian Spanish differs from the Latin American varieties in pronunciation and other details. However, all Latin American varieties are easily understood by Spaniards, and are recognised as different versions of Spanish by the Royal Academy of Madrid, the barometer for all things Spanish language. While some Spaniards believe theirs is the more 'pure' version of Spanish, most Spaniards recognise the reality that there is no 'pure' Spanish, even within their own country. While the differences in spelling are virtually non-existent the differences in words and pronunciation (as well as some aspects of grammar) between "Spanish-Spanish" and "Latin-Spanish" are arguably bigger than those between "American" and "British" English.
French is the most widely understood foreign language in the northeast of Spain, like Alquezar and Cap de Creus, as most travellers there come from France.
Locals will appreciate any attempts you make to speak their language. For example, know at least the Castilian for "good morning" (buenos días) and "thank you" (gracias).
I'm so pregnant
Many English words have their origins in Latin, which makes it easy for English speakers to guess the meanings of many Spanish words. However, Spanish and English also have a number of false friends that one needs to be aware of to avoid embarrassing mistakes.
-embarazada - pregnant; notembarrassed
-suburbio - slum; not suburb
-preservativo - condom; notpreservative
-bizarro - brave; not bizarre