The most widely understood and spoken language on IRC is English. However, as IRC is used in many different countries, English is by no means the only language. If you want to speak some other language than English (for example with your friends), go to a separate channel and set the topic to indicate that. On the other hand, you should check the topic before you move to a channel to see if there are any restrictions about language. On a non-restricted channel, please speak a language everybody can understand. If you want to do otherwise, change channels and set the topic accordingly.
It's not necessary to greet everybody on a channel personally. Usually one ``Hello!'' or equivalent is enough. And don't expect everybody to greet you back... On a channel with 20 people that would mean one screenful of hellos. It's sensible not to greet, in order not to be rude to the rest of the channel. If you must say hello to somebody you know, do it with a private message. The same applies to goodbyes.
Also note that using your client facilities to automatically say hello or goodbye to people is extremely poor etiquette. Nobody wants to receive autogreets . They are not only obviously automatic, but even if you think you are being polite you are actually sounding insincere and also interfering with the personal environment of the recipient when using autogreets. If somebody wants to be autogreeted on joining a channel, he will autogreet himself.
When you enter a new channel, you should listen for a while to get an impression of what is been discussed. Feel free to join in, but do not try to force your topic into the discussion if that doesn't come naturally.
Remember, people on IRC form their opinions about you only by your actions, writings and comments, so think before you type. If you use offensive words, you'll be frowned upon. Do not ``dump'' to a channel or user (send large amounts of unwanted information). This is likely to get you kicked off the channel or killed off from IRC . Dumping causes network ``burbs'', connections going down because servers cannot handle the large amount of traffic any more. Other prohibited actions include: