R leaflet Language of the map : Can we specify to use English language

What you see on the map are Tiles, which are plain images generated by the Tile Server you specify (or use by default).

In your case, the Tile Server is OpenStreetMap: you will get the exact same tiles as on openstreetmap.org website (see for example: http://www.openstreetmap.org/#map=4/28.46/86.92)

As these are images, you will understand that you cannot change them.

Now you can change the Tile Server until you find one that serves images with names in the language you need.

Have a look at http://leaflet-extras.github.io/leaflet-providers/preview/ for example.