Good idea on learning new languages! German is my native language, I'm (pretty much) fluent in English, and I speak/understand French and Japanese on a basic level. While I did a lot of studying for Japanese on my own, all the other languages I pretty much learned at school.

I can show you my road map for my past experiences with learning Japanese, but I can tell you that you have to invest a loooooot of time to be conversational level in Japanese and just for vacation there's only two things that make sense: First, learn Katakana and get good at interpreting them. It's only like 50 syllables, but they will be a great help on a vacation, especially in super markets for example. But even after learning them, you have to practice them and read 200-300 examples to get a feel for them. The good thing is: Japanese people use them mainly to "fake" the English language. For example: Terebi. If you know that R gets pronounced as L and B more like W, you can get closer: Telewi. Television. TV. You know probably see where I'm coming from when I say you need practice. But with just 3 syllables you know and you're able to read TV. In the super market, many of the bags say Miruku. What could that be? Us are often silent, Rs are Ls. Let's try it again: Milk. Ahh, you're currently shopping for milk! You can get a grasp of those Katakana pretty quick, because there are only like 55, and in today's age with the globalization and the internet, often times the Katakana will give you a context. So you will know a loooooot of words (check this for example: http://infohost.nmt.edu/~armiller/japanese/kanaloanfr.htm ). Of course they're not always used, but for me the Katakana were more than often a life-saver for context and are very little effort to learn.

My second suggestion would be, that you get a travel translation book with English and Japanese next to each other, where you can point to phrases for conversations in Japanese. With those little helpers, you're prepared for almost any emergency situation. You will be able to talk about/ask for directions, order the proper food in a restaurant, find bath rooms, and will be able to communicate with a doctor about almost any body part. Yet it's lightweight and easy to carry around. Very, very handy. I didn't really need it myself, but I felt safer having one with me, just in case.

If you're still convinced you want to go the full route, I'd suggest that you start with both Kana alphabets, which will already keep you busy for a while, you absolutely have to be fluent in them and be able to read them without thinking. Then you should grab a book (similar to Tae Kim's website for example), that will teach you the basics of Japanese grammar. When I was at that point, I started visiting weekly japanese classes for three semesters, which helped quite a bit to remember the stuff still (because I have context and associations with the grammar/lessons). After like 2 semesters of weekly lessons, we were at the point of learning the end boss: The Kanji. Those little bastards make the difference between people who do Japanese for fun and people who are serious about it. And there's no recipe for them. Some learn them faster in class room environments, some learn them faster by locking themselves up in a room for months and do nothing else. Some use spaced repetition systems (I know Anki (for offline learning) as a software for example). Some use context based spaced repetition where they try to trick their brain into associating something with it (for example Wanikani). For everyone there's a different approach and there's not general recipe for everyone, but one thing all have in common: it's a very looooong and very hard way which requires a fuckton of discipline (unless you're gifted of course). I'm not saying it's impossible, it's obviously not, but you pretty much need to dedicate your life (like 2-3 hours a day) to Kanji for months. And the even harder part: if you don't use them regularly after your vacation, they'll be gone faster than you know it. And be honest, how much would you be able to keep them in your brain afterwards? Katakana + translation script will be a couple of weeks of practice and in my opinion are perfect for a vacation. You're prepared, but obviously not conversational. But if you put alllll that work into Japanese and after a couple of months after the vacation you start forgetting all of it, that will be a lot of wasted money and time (and blood and sweat and tears).

I'm not trying to deter you from speaking another language, it's awesome, I'm writing in English right now, but I want to give you a realistic outlook on learning Japanese, especially "just" for a vacation. If you're still motivated and think you can do it, grab tables and Anki/Wanikani/any other spaced repetition websites and start learning the Kana. Once you're done with them, I can tell you already that Kanji are a hundredfold the work of the Kana, if not more. Needless to say, I never learned more than 200-300 Kanji at which point I could not put more time into my Japanese and without time and dedication, it's near to impossible. I was at peace with myself, knowing that this feat is just one that's too big for me. I was happy to know the basics of a beautiful language and I love visiting the country, but I found in the end I get happier when I put my time into other things, and so I bought a piano (which is getting dusty now goddammit).

Good luck with whatever choice you make, I hope it's the right one for you and an update in half a year would be interesting :)

Surround yourself with media in the language you want to learn and confront yourself with it daily:

* Read beginner-level books (jump over words you don’t know in the beginning as long as you understand the meaning of the sentence).

* Watch series & movies.

* Switch your computer’s system language.

* Switch your phone’s system language.

People have already added some great advice for your Japanese' sake, so I'm going to only chip in what I've learned myself.

It astonishes me that people grow to simply learn the "native word/sound -- foreign word/sound", while language is so much more than that. If one's own language is capable of producing words of similar meanings around the same subject, why do we assume that foreign ones lack it? So, learn the meaning. Learn what makes this idea of a thing this and not something else, and what other ideas of a thing there, and in what contexts can and shouldn't you use this particular idea. This is an important part of any language that people are missing for some odd reason.

Learn your own first. If you don't know what a particle is (in terms of linguistics), you will have a hard time with Japanese.

Sino-oriental languages are really easy to learn if you think about it in 'chunks'. Speaking is the easiest part.

Be more interested in social ladders than pronunciation. Don't call out native speakers for grammatical errors.

I speak Telugu and Spanish, the first of which I learnt at home so I can't really speak for that. However, I learnt Spanish over the course of 5 years in grade school and 1 semester in college, but I feel that my command of the language greatly improved after I started writing and speaking frequently through the help of language and composition classes. It seems like you're already planning to speak to native speakers so that's excellent, but make sure to note down any repeated errors you make while speaking so you improve over time. You might also want to find a local meetup of language enthusiasts so you can converse with nonnatives and learn their tricks to mastering the language as well.

I think you're doing great already! I went to Poland for my exchange semester at school, and being a native Russian speaker, I somehow thought I,d be able to pick up the language really fast so I didn't bother learning much apart from the tourist book sentences before I moved.

BIG MISTAKE

Because it meant I didn't have basic skills and I could not make proper Polish friends :( I had classes at university and went to weekly language exchange groups and only started to have a decent conversational level towards the end.

At that point, we had a drunken night with a bunch of Polish people and I swear I never improved so much as during that single night out. Because I could understand and maintain basic conversation and it was all effortless and fun! I realized that once you have enough to maintain to most basic of conversation, your rate of improvement increases exponentially! If only I had bothered to learn a bit before I left, I'm pretty sure I would have become almost fluent by the end of my 5 months stay there :( Huge regret.

Basically, just learn as much stupid scholarly classes as you can (Mike went to the gym. I study literature at university. Next summer, I plan to go work in Asia. etc) enough to understand people talking (watching movies is a great way to test how much you understand in regular speach). After that, it's on you to be sociable because there is no better way to learn than to shoot the shit with friends.