i18n: change the priority of where plugin_textdomain is hooked

Props @michelwppi:

"The modification is not perfect and the enqueuing in 'plugins_loaded' filters must set at level 100 and not 10 by default (too soon). Because other plugins can enqueue their own filter between 10 and the 100 level of jetpack 'Plugins_loaded'. (more logical because jetpack choose the 100 level)"

See original trac ticket:
https://plugins.trac.wordpress.org/ticket/1764