I am the author of Polylang But I noticed the same issue with WP Native Dashboard (and it probably exists with other multilingual plugins too).
How to reproduce?
* activate Jetpack 2.2 and a multilingual plugin
* switch the backend language to something different from the default one (normally set by WPLANG)
-> the WordPress admin interface (and generally other plugins) are correctly translated but not Jetpack.
At first glance, Jetpack loads its text domain in 'Jetpack::init' function hooked to 'init' action so multilingual plugins should have the time to switch the locale before the plugin text domain is loaded...
But "Jetpack_Sync::sync_options" calls 'Jetpack::init' too, as soon as the plugin is loaded and before any hook is fired, thus multilingual plugins cannot do their job.
Can "Jetpack_Sync::sync_options" wait for the hook 'plugins_loaded' to be fired?