I agree that French and German are good languages to learn if you are planning a move to Europe. As someone who has studied a number of languages, I tell anyone who is serious about learning a foreign language to get three basic reference books:
• A dictionary
• A verb book
• A reference grammar
The dictionary should be the biggest, fattest one that you can afford. It is important for it to show the various ways that a given word can be used. My Oxford Spanish Dictionary does precisely that. You need the verb book because verbs have so many forms; some are irregular and some are not. The reference grammar is like a dictionary for the rules of grammar and usage: it is generally arranged so you can look in the list of contents and find the particular rule you need to know about (for example, interrogatives, relative pronouns, compound tenses, etc.). In an ordinary textbook, these rules would be spread throughout the book, whereas in a reference grammar they would be all together in one section. With these three books plus the wonderful free resources of the Internet, you should have what you need to learn the European language of your choice.
Another useful practice to follow is to keep a alphabetized vocabulary list on your computer as you encounter new words and new ways to use them. I have one list for Spanish masculine nouns, another for feminine nouns, and yet another for nouns that can be either gender (words like azúcar and those ending in -ista). I have another list for adverbs, and others for the different kinds of regular and irregular verbs. You get the idea. Here is a sample entry from my list of feminine nouns:
BUSCA search, hunt || Enviaron a un grupo en busca de ayuda. They sent a group to look for help. || Anda en busca de marido. She's looking for a husband.
Be sure to find out how to use the computer to make any accented letters (or special characters like the “ß” in German) required by the language you wish to study. There are various ways to do this, some easier than others. With my word processor, I can shift between an English and a Spanish keyboard.
Finally, try to spend time with people who speak the language you wish to learn, preferably people who don’t speak Spanish very well, so you will be more likely to increase your skills by speaking in their language.
Even if you end up moving to an English-speaking country like Ireland, you may want to travel on the European continent, so whatever other language(s) you learn will not be time wasted.