The character encoding system in Java and C# is UTF-16. 

In C++ the default is UTF-8, other encodings are configurable:

aimms::IConfig* config = AIMMS::createDefaultConfig();
config->setCharacterSet("ASCII");

or

config->setCurrentLocaleCharacterSet();

And subsequently pass the config with aimms::openSession. 

For more information on character encodings, see the wikipedia page.