To set a specific language in a multi-site the language files must be available in the language directory, usually in wp-content/languages. So if you want to set the language to ja_JP a file ja_JP.mo must exist there. You can get the language files from the SVN repository.
If the issue is duplicating content to translate a site, it’s better to let a plugin do it, or build your own plugin to deal with it. There are not many of them, and I can cite +WPML, +Multilangual Press and -qTranslate. Related posts: How to change menu according to the language? How to setcookie … Read more
Install the plugin WP Native Dashboard. Then you can set one language for the front-end in your wp-config.php and each user can choose another one for the back-end. See Change language of comments template for details and a screenshot.
Xili language plugin not showing language flag [closed]
Hook in to add new post link wp admin?
I am not sure that you have provided enough information for solving this issue, it is unclear why and when your menu disappears and under what conditions. I am not sure why you are registering all the menus, when you probably need one. I can only advise you to use a plugin that manages languages … Read more
How to split existing blog into WordPress Multisites without breaking links?
There are two types of translations in the WordPress website: Label Translation in WordPress Core and Theme, for that gettext technique is useful (as mentioned by you .po and .mo files) Content Translations for that you will need to add multi-lingual plugin to handle such feature. Good Example of such plugin are: http://wordpress.org/plugins/qtranslate/ http://wordpress.org/plugins/multilingual-press/ More: … Read more
How to make a bilingual front-end post using the plugin WP User Frontend and Polylang or qTranslate?
I would question “obviously” here. I get rather cryptic string C as result on my Windows machine. Since WP isn’t actually using native PHP functionality for any localization and handles it via core code, my best guess would be that is coming from server configuration in your case.