"Congrats" is slang, but I think it should be accepted anyway. Thanks, added it, now it's accepted.
Technically yes: "Вітаємо в Києві!" = "Welcome to Kyiv!" (literally, "we greet you in Kyiv").
However, usually it would appear in 3.p.p. (i.e. "we") and with something after "вітаємо" :)
Otherwise welcome = "ласкаво просимо" (lit. "gently asking")
I think no, because for it to mean "welcome" it has to either have other words in front of it or be in 3 p.p. (Вітаємо!)
Would you say that to a person that just accomplished something? Because "congratulations" is also given as a possible translation.
Yes, its meaning can be "Congratulations!". It literally means "[I] congratulate [you]". And it's used to congratulate someone (not necessarily on an accomplishment, but yes).
As to the second meaning, it happens to also mean "to greet", so it would be "[I] greet [you]" as well and can mean "Greetings" or "Welcome".
Congratulations is it formal format? How I can say in informal relationship?