We literally say Nice to meet. The verb träffas is a deponent verb, read more about those here: https://www.duolingo.com/comment/6094592 Anyway the meaning of träffas is reciprocal, meaning that two or more people do something to each other, which makes it a little easier to leave out the object, since the verb in itself presupposes that two or more people are involved.
Edit: let me rephrase that. It's not just that you can leave out dig/er, in fact you can't use it with träffas. You'd have to put some preposition in between for that to work. This is because, as I said, the verb already in itself presupposes two or more people acting on each other.
It's not needed when you use träffas. You could say "Trevligt att träffa dig/er."
I think träffas is kind of intransitive verb and it can't have an object after it, as it should be translated to meat each other. Like in sentence:
Ve träffas utanför dörren.
I think this sentence is better understood as "it is nice that we meet each other", but this is not how one would say it in English.
It is the same with verb ses/hörs. It would be vi ses/hörs without any object after them.
No, not at all. Just trying to keep from deviating too much from the words in the sentence.
if i meet someone, should this phrase be used only at the start of the meeting, or is it also ok to use at the end of the meeting (e.g. after 3 hours conversation )?
in real life I never heard any swedish people using this.
The most common use would be if you are introduced to someone. But it could be used in past tense as well. "Det var trevligt att träffas igen efter alla dessa år.". "It was nice to meet you again after all these years-"
I was watching the show Bonusfamilj and the characters would say only 'trevligt' when meeting someone. Is it common to shorten this phrase in such a manner?