When Aang landed on Kyoshi Island in 99 AG, the foaming mouth guy went into a screaming fit and started foaming at the mouth before fainting and falling to the ground. When Katara and Sokka later returned to Kyoshi Island without Aang, who was being held prisoner in Chin Village at the time, the foaming mouth guy repeated his performance, only to be embarrassed when he realized Aang was not with them. He got up, wiped the foam from his mouth and walked away from the crowd, his shirt pulled over his face in shame.
The cries he makes are not dubbed over in the German or French language episodes, and he does not make any noise in the Dutch version.
There were originally plans to have a "foaming mouth girl" at the Revelation rally, but this plan was scrapped as the creators felt it undermined the creepiness of the event. The character is still present at the rally, but does not act unusually.
↑From older Avatar: The Last Airbender official site, originally on Nick.com (link). No longer updated.