Actually, they aren't that different ;) both are indo-eurbranc languages from the european branch. And yeah, they sound very different, but many words are very similar, because both languages have the same origin.
But think of this, Bisaya, and austronesian language totally unrelated to english apart for having some indo-european loan words, says the word two as "duha", similar to other indo-european language (deux, dos, tva, etc)
Спокойной ночи means good night, while добрый вечер means good evening. Going a bit more in depth than your question, good day would be used around noon, good evening would be used from about one in the afternoon to about sunset. After sunset, you would use good night. Thats how spanish does it; it may be different for russian. Feel free to correct me if it is.
A good night is always peaceful, calm and undisturbing. That’s what спокойный (спокойная, спокойное, спокойные) means. «Спокойной» is the genitive case for the singular feminine form «спокойная». «Спокойной ночи!» is short for «Желаю/желаем вам/тебе спокойной ночи!» (“I/we wish you a quiet night”). «Доброй ночи!» is also an option, but it is not used very often.
I wrote "By, good night." I missed the e. Got it wrong. Really? A common typo? I got another one wrong earlier because I spelled "out" instead of "our." Come on. I have nerve damage and it causes typos.
Also, how do we contact? I never want to type in Russian. That option needs to be available.
As an experiment I wanted to check if it would accept "bye, have a peaceful night". It did not even if in many cases duo insists on a literal traslation even at a cost of not sounding natural. Do you tjink such forms should be accepted or do we want to train in the phrase in the context without variations?
A literal translation is fine only if it clarifies structure or meaning. I'm not sure how helpful "have a calm/tranquil night" is here. There are two things there.
1) peaceful/calm/tranquil might be acceptable, but they definitely shouldn't be the main translation.
2) "have a" is actually adding a layer not there in the Russian. If anything, the use of the genitive implies a different verb: желать, so if you really want to be literal, you should probably go with "I wish/bid you (a) calm night."