I can tell you about my language. It is really hard to start learning the Russian language, but you'll be able to master it with less effort. For example, English is very easy at the start, but it is really difficult for mastering in the later stages, right? Because I've never seen any students of English, that speak English perfectly. Whatever you guys think of it, I can't see here the language, which I see in the movies... :) In the Russian language the grammar is just incredibly complex, if you wonder.

Personally I think it would be better to concentrate on your second language more first but if you want a third language just for the sake of it, Spanish would probably be the most useful. If for your career, then maybe something more practical like Japanese. You can try some out first and then decide which one is the most interesting to you.

@M_rk_s You may think it is reasonably good, but I wouldn't say that. It seems you know a lot of vocabulary but there are lot of simple mistakes. That's why it all depends what she plans to do with it. If just for communication, people can get along with broken English, etc. but if for her career she'd need a higher level than your apparent C1 level. As for age, I don't think there's much difference about learning now compared to a few years down the road; it's not like she's a child. For me, having a fluent second language is more impressive than a few so-so languages, but again... It depends what her plans are.

@Matt. I do not agree that you should focus on your second language. She is 18, now is the time to learn languages. I think it are typical American cold feet, here. (Or to be precise, before someone accuses me of stereotyping, a typical statement from someone who comes from a geographical area where only one or two languages dominate.) Here in the Netherlands, every teenager doing medium to upper level high school studies German, French and English. Apart from that you can graduate in Russian, Spanish, Frisian and Arab. I did English, French, German and Spanish in the lower classes. I graduated in English, German and Spanish. And then my focus was even science, not languages. I am still alive, and my English level is reasonably good, if I say so myself.

It depends upon your interest if you're going to be practical Spanish will be the best. 

Yes, it is also good to learn the langauge of your neighboring countries like Japanese, Korean, Filipino, Bahasa, Thai, Tieng viet, and what not. But for me, If i will choose another language, I will choose a language that will benefit me. Having learned a language will not be beneficial if it just serves as an ornamental or merely for ostentation purposes.

I, as a Filipino, have chosen Spanish as my fourth language. 

Filipino (national language) , English, Kapampangan (Language in our province) and Spanish which I took up last year..

My professor told us that Spanish will be useful for it is widely used after the English language. There might come a time that Spanish will dominate the world. Who knows?