Law French & Law Latin
For background see 'The three languages of the common law' in Baker, The Common law tradition: lawyers, books and the law (Legal Hist. B167d) and the sources mentioned in the footnotes.
Baker’s Manual of Law French at KL44.BAK 1990 is the best guide for anyone needing to translate from the original.
Latin (in a post-classical form) was the usual language of writs and court rolls until 1731 (4 Geo. II, c.26).
Some examples of early legal dictionaries are available via links below.
First edition of Stroud's Dictionary (The judicial dictionary of words and phrases judicially interpreted) was published in 1890.