Translation of plugin in MU-PLUGINS directory not working

You need to use another action when in a mu-plugins.

add_action('muplugins_loaded', 'myplugin_muload_textdomain');

plugins_loaded action work only after active plugins and pluggable functions are loaded.

mu-plugins are not regular plugins and will not be loaded like them.

As you can see in the following link, mu-plugins are loaded before anything else. Actions Run During a Typical Request