Open up the full chat window by clicking on the arrow next to your input box. When it is opened, look for the cog at the top-left corner, and click it. When the settings box appears, uncheck the "hide non-English language input" option or equivalent, and it is disabled.
Please note that the option will not be there in sPVP, Heart of the Mists, or World versus World. This has been done so that players of any language can communicate in these more serious environments. Make sure you're not in these zones when doing it.