You can change the language used by Meebo Messenger in the sign in page. Meebo Messenger supports dozens of languages. Scroll all the way down the page and click the language drop-down menu - it will show your currently selected language. Once you sign in, you will see the text in the Meebo Messenger interface displayed in the selected language.
If you want to change the language after you have already signed in, you'll need to refresh the page (which will sign you out), change the language, and sign back in.
If you notice that there are translations for certain phrases that are missing or incorrect in a language you are familiar with, you can click help translate! next to the language selection drop-down menu. This will take you to a wiki page that contains translations for all supported languages. You can create an account on this wiki to add and modify the translations. Thanks to all the users who have already helped to make Meebo available to everyone!
Unfortunately, ICQ isn't the best about using non-ASCII characters. The standard English alphabet is pretty universal in the computer world. There generally aren't problems with it, but when you start using characters that aren't used in English you can run into problems. Basically when you transmit some letters over the network, some letters can be encoded in more than one way, like Russian and Chinese characters, for example. So when you have two people talking to each other using an instant messenger, it's important that the two instant messaging programs use the same schemes to encode the characters. Unfortunately the ICQ protocol doesn't do a very good job of that.