If browser language differs from book language, promote translation

We currently have some concerns with the quality of the translation which we want to get more info in via #1485 . But in the mean time, we can try to put a feature in place that allows for promoting the translation menu when it would be useful -- eg when the browser language is different from the book language.

In the UI, this would cause the translation icon in the sidebar to be promoted to be displayed at the top-level shortcuts, with a blue dot denote notifications. The first time (maybe all times?) this happens, a dismissable popover will appear next to the icon in the same shade of blue describing the feature and informing user of the feature, with a CTA that opens the sidebar. Future visits will show the blue dot and promote the icon, but not the popover.