It might not seem that way, but in a legal context, these rather ordinary words are expressing specific notions. The notions of fond/forme in French law find their English counterparts in the English notions of procedural/substantive law respectively.

French:
« Les parties choisissent d’appliquer la loi française au contrat, aussi bien à la forme qu’au fond ».
Suggestion in English:
« The parties agree that the contract shall be governed by French law, with regard to matters both procedural and substantive.”

You could use "ordinary" English, which would give similar meaning but you might be glossing over the specific notions to be read into this legal context.

http://www.juritravail.com/lexique/Aufond.html
"Au fond" est une expression utilisée, dans les plaidoiries des avocats, dans les jugements, dans les arrêts, comme dans les actes de procédure, pour annoncer qu'après avoir examiné les questions de forme notamment celles relatives à la recevabilité de l'action ou de l'instance (compétence, respect des délais, fins de non-recevoir), le juge aborde celles des questions qui lui ont été soumises par les parties touchant au contenu et au fonctionnement de l'ordre juridique, on dit aussi "le fond du Droit".

http://www.juritravail.com/lexique/Forme.html
La notion de "forme" des actes, des jugements et des arrêts se réfère à leur conformité aux dispositions légales qui fixent les règles qui doivent être suivies pour la validité de leur établissement et de leur rédaction (validité externe). En revanche les questions qui touchent à la recevabilité comme à la compétence ou à la forclusion, constituent des questions de fond.
[…]

http://en.wikipedia.org/wiki/Substantive_law
Substantive law is the statutory or written law that defines rights and duties, such as crimes and punishments (in the criminal law), civil rights and responsibilities in civil law. It is codified in legislated statutes or can be enacted through the initiative process.
Substantive law stands in contrast to procedural law, which is the "machinery" for enforcing those rights and duties. Procedural law comprises the rules by which a court hears and determines what happens in civil or criminal proceedings, as well as the method and means by which substantive law is made and administered.
However, the way to this clear differentiation between substantive law and, serving the substantive law, procedural law has been long, since in the Roman civil procedure the actio included both substantive and procedural elements (see procedural law). [1] Notes and references[edit]
^ Reed, Shedd, Corley, and Morehead, The Legal and Regulatory Environment of Business.

http://en.wikipedia.org/wiki/Procedural_law
Procedural law or adjective law comprises the rules by which a court hears and determines what happens in civil lawsuit, criminal or administrative proceedings. The rules are designed to ensure a fair and consistent application of due process (in the U.S.) or fundamental justice (in other common law countries) to all cases that come before a court. Substantive law, which refers to the actual claims and defenses whose validity is tested through the procedures of procedural law, is different from procedural law


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Note added at 2 hrs (2013-07-09 09:27:38 GMT)
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The distinction to be made, therefore, is not to do with theory and practice.
Amaury has suggested "form and substance" which is probably as close as you can get to in using layman's language to convey the notions of procedural and substantive.

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Note added at 2 hrs (2013-07-09 09:31:26 GMT)
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I'd suggest avoiding validity and application which are a little off-target in terms of meaning.

Further, it would be quite normal in legal English to use "govern" in there somewhere to convey the notion of "application".

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Note added at 2 hrs (2013-07-09 09:38:04 GMT)
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Amaury is the French guy who was first to answer. I agree with his reading of it and like his "form and substance" which are probably better than "procedural/substantive". Just reminded me of some of my very first law lectures back in Oxford thirty years ago now!

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Note added at 2 hrs (2013-07-09 09:40:16 GMT)
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In certain contexts, circles, milieux, "procedural/substantive" may be a bit pretentious. Depends on the register of the rest. "Form/substance" are simple words, the underlying notions remain the same and the plain English brigade will be smiling all the way to court!