Sharing: When switching site language, switch sharing label's language automatically

When switching a site's language, the Sharing label remains in the language the blog was originally created.

Ideally, when a user switches the site's language, the label's language should change automatically to save the user from having to figure out where to change the text.

#### Steps to reproduce the issue

1. Create a site with the language set to English US.
2. Go into Settings > General and select a language for which Jetpack is fully translated – such as French France (FR_fr) – and activate it.
3. Activate the Sharing module and set the sharing icons to display on posts.
4. Visit a post on the front end.

#### What I expected

The label above the sharing icons should be in French.

#### What happened instead

The label above the sharing icons was still in English.

Replicated in multiple themes in multiple languages, here are two examples:


The only way to modify the sharing label's language at this point is to manually go into Sharing Settings and adjust the text by hand there:

**WP Admin**


**Calypso**


No written user report, I noticed the issue while helping a user in person at a local WordPress event.

Reproduce the issue through Settings > General, the Sharing module, and the post front end, checking both WP Admin and Calypso settings. Trace how the sharing label is stored and localized when the site language changes; done means the label follows the selected site language without manual editing, with the reported English-to-French case behaving as expected.

Written by the indexing model from the issue text.