I don't do literary translations, but I have done plenty of technical translations from Japanese to English. The biggest mistake I see novices make is trying to do a literal, word-for-word translation. It never works because the language structures and cultures are too different.
The key to a good translation, in my mind, is understanding the source language and the content well enough that you can write a good target translation that reads as though it was written in the target language to start with. The point of translating something is to convey an idea, not a list of vocabulary words and grammar structures.
Were you paid for the translations? I'm interested in learning more about technical translations as part-time work.
I've translated some articles from Russian to English, though I don't know Russian and instead relied on Google Translate combined with domain knowledge for phrases translated incorrectly.
The key to a good translation, in my mind, is understanding the source language and the content well enough that you can write a good target translation that reads as though it was written in the target language to start with. The point of translating something is to convey an idea, not a list of vocabulary words and grammar structures.