I need to setup a blog using wordpress engine but i need it support multi languages (3 languages), currently i am thinking of using custom field to identify the language of
I fi i remember well, wordpress uses a gettext "customization" library to translate itself. I think you can make some options based on a session variable, about the translation files you're going to read.
Try using plugin wordpress WPML.
I have used the xlanguage plugin and I am happy with it. xlanguage site
I've tested 2 plugins that both looked very promising: qTranslate and Gengo
qTranslate works by adding inputs for all used languages for each post, Gengo used separate posts and links them in a "post Y is a translation of post X" way, which I preferred much (because it allowed me to not translate everything) and it worked fine for me.