Currently i18n documentation exists, but it's not quite helpful. Almost all of them say:
Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details
Here's an example extraction...:
i18n/qqq.json
"sitemetrics-anon-edits-month": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-avatars": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-avatars-day": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-avatars-month": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-awards": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-awards-day": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-awards-month": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details", "sitemetrics-blog-stats-header": "Refer to the [[mw:File:SiteMetrics.png|extension screenshot]] for details",
https://github.com/wikimedia/mediawiki-extensions-SiteMetrics/blob/master/i18n/qqq.json