Hard to say when a French word became English, because people spoke franglais - they peppered their English with French words. And very little English writing survives anyway, especially if you discount later copies that might have been modernized by the copyist.
Routine formal knighting started about the mid 13th century, give or take. There are few early records, but people started appearing in legal documents with miles or chivaler after their names. It was going out of fashion by 1400.
Earlier, a few very important people were knighted by elaborate ceremonies - the forerunners of Knights of the Bath. One theory says this was a prerequisite for an earldom.
But "Sir" wasn't always linked with knighthood as such. That usage probably settled about 1600. A century earlier, you can find clergymen called Sir although they weren't knighted. In the Agincourt roll, a variety of people are called Sire or Msgr but it's hard to see what it signifies.
Going back as far as Wace (time of Henry II), sire was used for landowners - sire of this place or that. (Later, knighthood was never territorial - nobody was "Knight of" anywhere, although you see it all over the net.)
The obvious Latin translation of sire was senior, but they don't seem to have used that. They just slapped dominus on everybody from a squire to a parliamentary peer. Peers are often called Seigneur in early records of parliament.
But common genealogical practice is to ignore the imponderables and use Sir to translate the postnominal miles or chivaler or knt.