Learning 3rd language in 2nd or 1st language?

I learned my fourth language in Spanish and Greek.

preferably in your first.

If you are Dutch and learned English as second language, learning German through English would be complicating things unnecessarily. Some things for which there are words in Dutch and German do not have a word in English

It doesn't really matter. I grew up speaking English. I learned Portuguese to fluency as a second language living in Portugal for three years as a teenager. Then I learned Spanish to fluency living in Miami in my early twenties by, at first, building on Portuguese because of the greater similarities between Spanish and Portuguese than Spanish and English. It took me only about six months to get to full fluency (though, to be fair, I was working 70 hours a week with non-English-speaking Spanish speakers), becoming so fluent that when I took a college placement test in Spanish the next year, I was the first student in the history of the university to get a perfect score, so the director of the Spanish department called me in and interviewed me to see if I had cheated or if I was actually a native speaker and so not eligible to take the placement test for college credit.

It doesn't really matter because once you've learned a second language, you learn how to think in a foreign language and bypass that "translation" (the word is actually "interpret" when speaking as "translate" refers only to writing) step. By the time you learn a third language, you don't think of what you want to say in one language and then figure out how to say it in another but form the ideas in that language in the first place and then if you don't know a word, only at that point do you go searching your brain for a word in one of the other languages you know that expresses that same meaning in order to look it up in a dictionary or ask someone nearby who knows the language you're speaking in and knows one of the other languages you know.

I will say this, though, because Spanish and Portuguese are both romance languages and because hardly any of the time did anyone speak either English or Portuguese, when I didn't know a work, I'd try to figure out what the Spanish word might be on what the Portuguese word was. As a result of that, when I speak Portuguese now, despite the fact that I'm now so fully fluent in Spanish that I get mistaken for a native speaker, it tends to muddle my Spanish when I switch back to Spanish and I find myself dropping in Portuguese words here and there and it takes me anywhere from 5 minutes to half and hour for that to stop happening. 

I KNOW it doesn't matter because I have since learned French and Italian, though not nearly to the level of Portuguese and Spanish. Your second language is the hardest to learn. That's because there's a certain skill to learning a language, a skill that you now already have and have already become good at when you get to your third, a skill you're even more expert at when you learn your fourth. A big part of that skill is learning NOT to do what you describe but learning to actually from the beginning form thought in a manner so that you don't think first in one language and "translate," as you put it, into the language you're learning but forming thought within the mantle of that language you're learning and then only seeking out what you mean to say in another language if you don't know a word for what you're thinking of or don't know how to describe it to whom you're talking to in that language so that they can provide it to you in that language.

The best way for doing this is by frontloading learning vocabulary before anything else. Second, verb conjugation. A method of vocabulary that helps you ideate in the language you're learning without resorting to another of your languages is to use Post-It notes to label things in your surroundings with that foreign word so that the image of that thing is associated with that word rather than an English word or some other word not in that language. Another method is to keep a standard dictionary in that language handy in addition to a foreign language dictionary handy, so that if you hear words you don't know, you seek out their definition in the language you're learning instead of seeking it out in a dictionary that's going to provide you the definition in another language you know.