What is right way to modify a node field in a multilanguage system (LANGUAGE_NONE or $node->language or ...)
field_language() returns the field's display language. You should use code similar to the following one: 

$language = field_language('node', $node, 'body');
do_something_with($node-&gt;body[$language][0]);


Of course, note that field_language() will determine the right display language ('en' or LANGUAGE_NONE or whatever); if you're modifying fields you might want to modify more than one language.