Allow language to be set via option

[liedekef]

Currently the language of the browser is used to show the overlay text on the map, but some people do have a basic browser in 1 language they can't (or don't bother to) change.
This would be useful in sites where the js used to show the map is built dynamically (and the user language choice is taken into account). Personally I'm thinking about a number of wordpress site where I use leaflet as the tool to show maps.