I have created a custom post types and added some custom metaboxes and now I am wondering witch translation plugin I use for my site?
I don't have experience with none of them, so I don't know who will support my custom metaboxes and who will not.
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take a look at Internationalizing Your Plugin codex entry to get a better understanding of WordPress Translations.
and read I18n for WordPress Developers after that you will know that if you coded your custom post types and metaboxes the right way with Translatable strings , all plugins work in the same way of looking for these strings.
and i would suggest using Codestyling Localization plugin which create a UI for translating plugins and themes inside WordPress admin.
You will have to translate your plugin yourself: http://codex.wordpress.org/Translating_WordPress
WPML fully supports custom post types, custom meta boxes, custom taxonomies and more.