When considering pre and postnuptial agreements in an international context one must consider whether the jurisdiction, which will be enforcing your agreement, will apply their local laws to see if it is valid, or the law of the jurisdiction where the agreement was made. It is therefore important that you consult with attorneys in whichever jurisdiction you might seek enforcement. Not all countries have the same rules on validity and enforcement. Washington State will enforce them if they reach certain fairness standards. See In re Marriage of Matson 107 Wn. 2d 479,730 P.2d 668 (1986) for the prenuptial agreement two-prong test for validity. For postnuptial agreements, see, for example, In re Marriage of Zier, 136 Wn.App.40,147 P.3d 624 (2006) review denied,162 Wn.2d 1008 (2007) and the earlier case of In re Marriage of Hadley, 88 Wn.2d 649,565 P2d 790 (1977).
It is clear from reading the Washington cases that, in general (there are refinements to this rule) that an agreement does not have to be substantively fair to be enforceable, if the procedure leading up to the agreement was fair. It is also reasonably clear in Washington, that even if the agreement is based on something other than local family law, that the same analysis will be applied. See, e.g., In re Marriage of Obaidi, 154 Wn. App. 609, 226 P.3d 787, review denied, 169 Wn.2d 1024 (2010). Obaidi, involved a prenuptial agreement (a mahr) based on Islamic Law. The couple signed the agreement in Washington during an engagement ceremony. The Washington Court of Appeals found it to be invalid as there had been no meeting of the minds on the essential terms. Id. at 617.