I was fighting a Sailor NPC on Route 222 and his Gastrodon used Hidden Power and it went like "The foe's GASTRODON used Hi(Then the text was interrupted and replaced with 11 spaces of glitch text!)". I have used an AR on this Gamecard, even though I stopped a long time ago. I think that probably influenced it somehow, but WHAT EXACTLY HAPPENED???
This was definitely caused by your AR.
I did a bit of searching, and this glitch has been reported nowhere but here. If this was a glitch that was caused by the game programming, it would have turned up on Bulbapedia's List of Glitches in Generation IV. Either the likely even of your AR causing this or other tweaking/hacking the game could have lead to this problem.