You can join channels by typing "/join (channel name)" and leave them by typing "/leave (channel name)" In addition to this, each channel is designated a number. You can replace the (channel name) with (channel number) when you want to leave a channel by typing "/leave (channel number).
To talk in the common channels, press /(channel number) for that channel. (Example: /1 will allow you to type in the General chat channel).
By default, the General, Trade, and Local Defense channels will update as you move from one area to another, while World Defense will stay static as it covers all areas. If you create additional channels, they will be designated channel numbers in sequential order.
Click on the "Chat Bubble" on the chat box tool bar, then highlight "Language" with the cursor and then a list of which languages you know will appear. From there you can click on and choose which language you prefer to speak.