Products that are localized to international markets often face domestic competition, which makes it critical for the localized product to blend seamlessly into the native language and cultural landscape. The cost of a localization effort can be significant. Once you have the strings translated and the GUI updated, localization testing should be used to help ensure that the product is successfully migrated to the target market.
In addition to verifying successful translation, basic functional testing should be performed. Functional issues often arise as a result of localizing software. Don't risk the time and effort spent localizing by not performing adequate Quality Assurance.