The Mega that you recieve will probably be decided on the country your console is set to.
The 3DS doesn't know in which country you live; rather it believes in what you input in your profile's country section. If I live in India and have my DS set to Australia, I will encounter Mega Glalie. Likewise if it were set to Japan, I would encounter Mega Steelix.
EDIT: So far, the Pokemon that will be encountered in USA hasn't been decided really. Only information I found so far was Mega Glalie for Australia, Europe and South Korea and Mega Steelix for Japan.
EDIT 2: USA has been confirmed to get Mega Glalie.
Hope I helped!