I'm a (mostly self-taught) student of Spanish and Japanese, two radically different languages in terms of vocabulary, writing system, grammar, geography, influence, culture, history, etc. Now that I've "officially" started studying Japanese in university, I've had a hard time keeping up with my Spanish skills. Hoping to dive back in, what are some of the 1) most common, and 2) most effective strategies for studying two unrelated languages?
For people who want to learn more than one foreign language at a time, choosing two unrelated languages, such as Japanese and Spanish, is a good idea because it reduces the confusion factor. However, it is still a challenging task, for example, because people typically have limited spare time after work or university-relate activites, because one of the languages is particularly difficult, or for other reasons.
The first thing to do is to work on your time management skills. As Donovan Nagel says, "If your time management skills suck then forget it".
Donovan Nagel also recommends focusing on one language at a time for a few days instead of switching between them duriing the day. (This is fine if you learn languages in your spare time, but a university student will need to work on their primary foreign language—in this case Japanese—at least five days per week.) The rationale between Nagel's advice is to allow short periods of immersion in each of the languages.
Nagel also recommends using a diary to allocate times of the day and week to each of the two languages. He adds, "Allocate times where you tell the rest of the world to piss off and leave you alone". You need to be ruthless with your time. Shannon Kennedy also mentions this: maximising study time, diligently sticking to a routine and a predetermined set of resources.
Another important aspect is motivation: you need to remember why you want to learn both languages. You should explicitly state the goals you want to achieve with these languages. (A purely emotional goal such as strong fascination is OK, as long as that fascination exists.) For people who are considering to learn two languages simultaneouly, Nagel adds, "Before you start, picture yourself 6 months, 1 year, or a few years down the track and answer honestly whether or not you’d still have the same passion and motivation to keep going with it."
Nagel also addresses the issue of fatigue:
When you go through periods of boredom or fatigue with one language (it happens to everybody), instead of not being productive at all you’ve got your other language/s to focus on for a while.
(Shannon Kennedy also mentions this.) For the current question, this is a bit difficult, because fatigue with Japanese would quickly lead to a serious backlog.
Shannon Kennedy also suggests waiting until you're stronger in one language before you add another. Bill Price gives the same advice; he found that for him "a solid A2/B1 is a perfect time to introduce a new language to the mix."
Some people suggest changing up where you study when you switch between languages. Shannon Kennedy does not do this; instead, she study wherever she is with whatever materials she has at hand.
Apply the Pareto principle–also known as the 80/20 rule– to your language learning activities: focus most on learning activities that bring the greatest benefits. This is definitely possible for the non-obligatory language (in this case, Spanish).
One thing to do if the second foreign language is not obligatory is focusing on materials that require less effort, e.g. listening to Spanish song or doing extensive reading with materials at your level (e.g. graded readers), instead of focusing on learning activities that require you to actively learn vocabulary and grammar. The advantage of this is that it reduces the risk of fatigue.
When I was studying two languages at the same time, I always tried to be at least a little proficient in one before adding another. That way, I could use them together to study, and train myself out of going through English!
So, I took Spanish first. When I was in second year (secondary) Spanish, I added first year French. This was not a great idea as the vocabularies were too similar and too dissimilar. The similarities meant that I got bored with French easily. The differences meant that I could not fake my way through by leaning on my Spanish.
Next, when I was in Spanish 3 and French 2, I added -- Hey! I was young, energetic, enthusiastic, and stupid! -- German 1. This was actually much better. I was stuck in my bad habit in Spanish. I translated everything! I was closer to matching vocabulary to idea in French, because my Spanish was only a little bit stronger than my French. I came closest to matching German vocabulary to idea (at that time) because I tried to use French if I felt the need to translate.
German and French were so unalike that I was able to use each of them to build up the other. I began taking my notes for German class in French, with recourse to Spanish if need be, but never English. This worked well for me.
My results were: -My French became fluent long before my Spanish did. -My Spanish stayed fluent with disuse longer than my French did. -I use them, practice them, both now. -My German became automatic rather than fluent. -One day, I took a test in German and realized that I did not really understand lots of what I was writing on it. -When I got it back, I had scored 99% -- an "A". -I dropped German the next semester. <-- This was another stupid thing I did. If I had persevered, I think I could have done truly well in German eventually. Ah me.
I went on to Near Native Fluency in both Spanish and French. I went on to graduate school in Spanish Linguistics.
I have since gone on to learn: Portuguese and Italian I can read in Romanian with time and a good dictionary. I am now studying: Biblical Hebrew and Norwegian. My Norwegian skills have grown leaps and bounds due to my previous work in German.
So, there is very little theory here, but lots of practice. My family hated me at one time because I had most surfaces covered with words in other languages (the window had tags that read: "la ventana" "le fenetre" "das Fenster") and they were written on colored paper! I hope this helps you with your situation.
I am attempting this now. I am learning Spanish and Hungarian; however, I am not setting the same priority on each one. In that way, they don't compete for my attention and emotional energy as much as if I were to be driven by the impulse of the moment.
Learning Spanish is currently my main priority. It is where I dedicate the most time. Hungarian is a longer term goal (which it would be anyway because, wow, Hungarian!). For me, it seems to work to have a target number of hours for each language per week, and those targets are very different. I spend more than twice the time on Spanish than I do on Hungarian -- although I am taking weekly lessons with instructors for both.
As my Spanish solidifies, I anticipate that I will increase the time on Hungarian.
It is very difficult to learn two different languages at a time. If you are proficient in any one new language then, learning other language is easier than the first. The most common way to learn any language is understanding, practice. In your case, no strategies I have to share because Spanish & Japanese have different structure, vocabulary, writing style.