The language a player's character speaks, for those with more than one, can be changed by going into the chat menu. Clicking on the "Chat Bubble" on the chat box tool bar, then highlighting "Language" with the cursor, will make a list of known languages to appear. From there, the player can click one to choose which language they prefer to speak.
It may be worth mentioning that when the language is changed, it only affects say, yell, guild, and party chat. General and whispers will be unaffected. Also, it tends to irritate a player's guild when they speak their "native" language.