I was having trouble with the same thing. Don't worry - it's simple to return to the English input method - once you know how!
Go to compose a message. Where you have the writing saying "Type to compose", touch and hold the screen. A box will appear saying "Edit Text. Input Method". Tap where it says "input method" and you will get three options (if you haven't downloaded any extras from anywhere else) - Standard keyboard, Japanese keyboard, and Android keyboard. Just tap the one you want - Standard keyboard and Android keyboard are both English, so see which you prefer. Just go through the same process to change again.
Hope this helps. Let me know if it's unclear.
Simon
