At the moment, I am studying Mandarin and Japanese at the same time. I get to take a class for Mandarin, I have to buy products to study Japanese.
But it's not enough! I want to study another language, so I've decided to come to you guys for suggestions.

Here are languages I've considered:
German
French
Latin
Korean
Esperanto
Hebrew/Greek

I trying to base my decision on what won't take too much time, but also what will be challenging and fun. I'm looking to use every language I learn, so I want something practical. I also don't want something extremely difficult. What do you think would be the best to learn? I want your honest opinion, it doesn't have to be unbiased.
Also, I've considered relearning Spanish, would that be good? I don't really think Spanish is that fun.

Awaiting your suggestions!

Go for the East Asian trifecta: learn Korean. Then you will never be sure how to pronounce any given Sinograph for as long as you live.

You used a colon where you needed a semicolon.
Any real advice?

Learn some manners?

=D please stop picking on each other
I often heard that it could be hard learning Korean when you are learning Japanese, but I kind of like this language. I also really like German (maybe because I'm german) so I would recommend one of those two languages by my personal taste.
If you want to go for something easy, take Esperanto.
Ah and I wouldn't recommend learning Latin, 'cause you have no one to practice and if you want to study a language because of great literature, I think german is a better choice than Latin.

Choose the language you like

gnunix

These are my own considerations. I don't know if you have any special attachments to these languages.

My advice would be to scratch Latin off your list, unless you are planning on becoming a priest.

Because you grouped Greek and Hebrew together, I'm going to assume it's for their biblical usage. If I'm right, I'd scratch them off your list right now. I'm not sure why, but the majority of people exploring a second language flock to these two; I did. (They rarely go very far before being dropped.)

It's estimated that the number of people who can use Esperanto fluently is between 100,000 and 300,000. "Small" minority languages like Welsh and Catalan have ~700,000 and ~7,000,000 speakers respectively. I think the idea of Esperanto is neat but not enough people speak it. The majority of esperantists probably speaks English, Chinese, Korean, or Japanese too. Scratch it off the list.

Now you have three to choose from: Korean, French and German.

Korean could work because you seem to have an affinity for the Far East.

French is probably the easiest language on your list to learn and has the widest geographical range.

German is also good and is the most spoken language in Europe, I believe.

@gnunix: I heard the average Japanese video game takes three times as long to translate into Korean as the next longest language translation period.

P.S.
I hope my semi-colon and colon usage is to your liking.

What's your source for that? Because it reeks of urban legend. It contradicts everything I've heard and read from Korean-Japanese bilinguals and my own experience. In fact, the grammar of these two languages is so similar, it's routinely possible to do morpheme-for-morpheme translations between them. I would expect Korean <> Japanese machine translation to be less lossy than it is for most Romance language pairs. (Unfortunately, you can't test this with Google Translate since it apparently translates these languages into English first rather than directly into each other.)

Maybe it would be best to just stick wth two languages at this point and see where that takes me.

I've been considering Romance languages lately, and have been curious as two what it would be like to learn Italian.

Ugh, decisions, decisions.