Pseudolocalization (or pseudo-localization) is a software testing method used for testing internationalization aspects of software. Instead of translating the text of the software into a foreign language, as in the process of localization, the textual elements of an application are replaced with an altered version of the original language. For example, instead of "Account Settings", the text may be altered to display as "!!!Ǎ¢ƈôΰлţ §℮τţĭπꞡş !!!".

These specific alterations make the original words appear readable, but include the most problematic characteristics of the world's languages: varying length of text or characters, language direction, fit into the interface and so on.

Traditionally, localization of software is independent of the software development process. In a typical scenario, software would be built and tested in one base language (such as English), with any localizable elements being extracted into external resources. Those resources are handed off to a localization team for translation into different target languages. The problem with this approach is that many subtle software bugs may be found during the process of localization, when it is too late (or more likely, too expensive) to fix them.

The types of problems that can arise during localization involve differences in how written text appears in different languages. These problems include:

In addition, the localization process may uncover places where an element should be localizable, but is hard coded in a source language. Similarly, there may be elements that were designed to be localized, but should not be (e.g. the element names in an XML or HTML document.)

Pseudolocalization is designed to catch these types of bugs during the development cycle, by mechanically replacing all localizable elements with a pseudo-language that is readable by speakers of the source language, but which contains most of the troublesome elements of other languages and scripts. This is why pseudolocalisation is to be considered an engineering or internationalization tool more than a localization one.

Although the pseudolocalization technique has been used at Microsoft since the late 90s, it was made available to developers as a feature during the Windows Vista development cycle. The type of pseudo-language invented for this purpose is called a pseudo locale in Windows parlance. These locales were designed to use character sets and scripts characteristics from one of the three broad classes of foreign languages used by Windows at the time—basic ("Western"), mirrored ("Near-Eastern"), and CJK ("Far-Eastern"). Prior to Vista, each of these three language classes had their own separate builds of Windows, with potentially different code bases (and thus, different behaviors and bugs.) The pseudo locales created for each of these language families would produce text that still "reads" as English, but is made up of script from another languages. For example, the text string

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