Having lived in Texas for a time as a small child, I picked up some basics of the language. However, it wasn't until my 2013 roadtrip in which I spent eight days in Spain that I took a strong liking to it. While not as traditionally 'romantic' sounding as French or Italian, I adore listening to the enthusiastic discussions of Spaniards, where its vibrancy is tinged with an air of comedy (I can't explain why it sounds comical to me exactly).
Not only do I like the sound of it, but Spanish is a widespread language. As a traveller, it would be a dream to be able to speak with locals not only in Spain, but also in much of Central and South America.
Unlike with Spanish, I don't have an affinity towards listening to and enjoying the sound of Croatian. The main reason I want to learn the language is that I fell head over heels for the country (its warm beaches, its friendly inhabitants, and the delicious traditional foods) and feel like it would be a place I could happily retire.
As a bonus, Croatian is strikingly similar to other languages of the former Yugoslavia, and speaking Croatian would mean being able to get by in Slovenia, Bosnia and Herzegovina, Serbia, Kosovo, Montenegro and Macedonia. So as with Spanish, it's a useful language to learn if you want to be able to use it in several countries.
Perhaps predictable, but French is a language I want to learn simply because one day I would like to live in France. I'm always a bit put off by how tricky the pronunciation is. Consonants at the end of words are often lost into the void, meaning that if you're not paying close attention to the context, a word could easily be singular or plural (and that's one of the tamest examples). For all its silent-letter flaws, and probably thanks to them, French is a very beautiful language. French is also spoken in several African countries, part of Canada and various Caribbean and Pacific Islands, so that's also an added bonus as a traveller.
I just love Malay. Many of the nouns bear a crazy similarity to English where the words are made more simple (and better). For example:
Boutique = Butik
Police = Polis
Immigration = Imigresen
Tea = Teh (Not necessarily better, but still awesome! Teh tarik is the shit.)
Not only am I inexplicably drawn to Malay, but the ties to English mean that it would potentially be easier to pick up than many other languages. And, of course, I greatly enjoy Malaysia and want to visit again and again.
One fascinating language is Esperanto. Devised in 1887 by a Polish physician, Esperanto was created to be an international means of communication (the word originates from Latin sperare "to hope"). He took bits and pieces from English and many of the main European languages and combined them into a new hybrid for what he hoped would be global usage. Now it is the most widely spoken made-up language, in part because it is much easier to learn than the ever more ubiquitous English, and may be taught to young children as a way to teach them language-learning skills. Interestingly, Esperanto doesn't use slang, as that goes against Doctor Esperanto's intention for the language to have worldwide usage.
Honourable mention: Welsh
Have you been to Wales? If not, you may be completely oblivious to the fact that they even have their own language, let alone how utterly ridiculous it is. Unless I become cripplingly bored, and have used that boredom already to learn the languages above, I will not be picking up Welsh any time soon (so it doesn't really belong in this post, but it's so bizarre I had to include it!). To give you a little treat, here are some choice words and phrases for your perusal:
Welsh = Cymraeg
Please say that again = Dywedwch hynny unwaith eto, os gwelwch yn dda
Bon apetit! = Mwynhewch eich bwyd!
Which languages are at the top of your list? Let me know in the comments below!