Can you tell me which are valid characters for nickname.Only english letters and numbers?Is it possible to have nickname in other language?Thanks in advance.

A name can only contain the following characters:

A through to Z. (Lowercase and uppercase.)
0 through to 9.
`|^_-{}[] and \

And a name cannot start with a number or hyphen.

Note that certain networks have specific requirements for nicknames that other networks don't have. Also, some networks allow unicode nicknames.

That's what i'm asking,whether in some networks nickname can be with non-english letters.Under Unicode you mean letters in some other language than english and certain characters,right?

Yes. Some networks allow that, though I am not sure which ones they are as I don't use non-English networks.