This validation checks if a string contains characters that do not belong to the language of the string. Sometimes when you enter translations you might enter characters that do not belong the script of the target language. This might cause that the computer where the end user runs the applications of views the document can not show the characters. Some examples:
When translating Chinese translators might enter Simplified Chinese when Traditional Chinese is needed or other way around.
If your application is an Ansi application it is very important that translations only use script of the language. Ansi applications can not generally show mixed script text and it is very likely that some of the text that does not belong to the default script of the language is not shown correctly. In Unicode application you can mix scripts but event there on most cases translations using two or more scripts are mistakes and not by purpose.
This validation can be applied both to original and translated strings.