A 15th-century manuscript of an English translation by "M. N." of The Mirror was found by Mr. J. A. Herbert among a manuscript collection purchased for the British Library in 1911 and was shown to Evelyn Underhill. Other 15th-century copies were subsequently found in the Bodleian and the library of St. John's College, Cambridge, together with a Latin version made in the late 15th century by Richard Methley of Mount Grace, Yorkshire. A printed edition was edited by Clare Kirchberger, from those four manuscripts, and published by Burns Oates and Washbourne Ltd., publishers to the Holy See, in 1927, complete with Nihil Obstat and Imprimatur. 
The translation by "M. N. " included a number of glosses by him, and divided the text into divisions and chapters.
The French book that I shall write after is evil [i.e. badly] written and in some places for default of words and syllables the reason is away. Also, in translating French, some words need to be changed or it will fare ungoodly, not according to the sense. - from translator's prologue
For the 1927 edition, the mediæval text was used but with spellings updated, and occasional words replaced accompanied by footnotes with additional glosses.