8. A system comprising: a data processing apparatus; and a data store storing instructions executable by the data processing apparatus that upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a set of acceptable expressions, each acceptable expression being a string that identifies a value of a variable entity in a first natural language, each acceptable expression being associated with a canonical representation of the value identified by that expression; performing, using a first machine translator that translates expressions from the first natural language to a second natural language, machine translation on each acceptable expression in the first natural language to obtain a translated expression of the acceptable expression in the second natural language; associating the canonical representation associated with each acceptable expression with the corresponding translated expression in the second natural language; providing a set of training data for training a second machine translator that translates expressions in the second natural language that each include a respective translated expression to expressions in the second natural language that each include a respective canonical representation, the set of training data comprising the translated expressions and the canonical representations that are associated with the translated expressions; and using the second machine translator to translate a particular expression that includes a particular translated expression into a particular translated expression that includes a particular canonical representation.
8. A system comprising: a data processing apparatus; and a data store storing instructions executable by the data processing apparatus that upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a set of acceptable expressions, each acceptable expression being a string that identifies a value of a variable entity in a first natural language, each acceptable expression being associated with a canonical representation of the value identified by that expression; performing, using a first machine translator that translates expressions from the first natural language to a second natural language, machine translation on each acceptable expression in the first natural language to obtain a translated expression of the acceptable expression in the second natural language; associating the canonical representation associated with each acceptable expression with the corresponding translated expression in the second natural language; providing a set of training data for training a second machine translator that translates expressions in the second natural language that each include a respective translated expression to expressions in the second natural language that each include a respective canonical representation, the set of training data comprising the translated expressions and the canonical representations that are associated with the translated expressions; and using the second machine translator to translate a particular expression that includes a particular translated expression into a particular translated expression that includes a particular canonical representation. 14. The system of claim 8 , wherein the set of acceptable expressions further comprises acceptable expressions that are strings that identify values of a variable entity in a third natural language.