Libel and slander is a topic on which you can find complete libraries; a general overview is too complicated.
About the more specific questions you asked, here is a European outlook, specifically as seen by the European Court of Human Rights.
The truth is not an absolute defence.
First you should realize that you should be able to back up your claim if challenged in court, and the onus of the proof is on you. This may prove to be quite costly and difficult.
Furthermore, even if something is true, there may be reasons that it is improper to make it public under certain circumstances. A case that was taken to the European Court of Human Rights involved a Dutch amn who had (as I remember) during WW II notified the German authorities of the presence of Jews. He was convicted after the war. Many years later (1980-1990) a newspaper published about this, but it was found that although the allegations were true, publishing it now after so many years did damage his current good reputation. (I’m doing this off the top of my head, so a few details may be off).
And on top of this, even if it is the truth, the way you phrase it makes a big difference. A truth can be phrased in an injurious manner. For a non-legal example see the threads here in the Pit.
If you only tell people in the private sphere things, it will probably never come to court. I can’t remember having seen such a case. Such private conversation is protected by the right to privacy. However, if you make it a practice to warn complete strangers against a certain person, that would probably be seen as leaving the private sphere.
International aspects are very complicated. In general, however, you should be prepared to defend yourself in any country where your allegations makes an impact (i.e. are published, read, acted upon, where the subject of your allegations lived, where you live). Even if a judge would in the end rule that he has no jurisdiction, it might take a while and be costly as well.
Very generally speaking, you should not lightly go around bandying grievous or injurious language. Even if you may in the end prove to be justified, the costs to get there can be enormous.