use Android Lint,
It specifically checks for just that case.
Summary: Checks for incomplete translations where not all strings are translated
Priority: 8 / 10 Severity: Fatal Category: Correctness:Messages
If an application has more than one locale, then all the strings declared in one language should also be translated in all other languages.
By default this detector allows regions of a language to just provide a subset of the strings and fall back to the standard language strings. You can require all regions to provide a full translation by setting the environment variable ANDROID_LINT_COMPLETE_REGIONS.
if you are using a recent ADT, you can run it directly from eclipse by right clicking your project > android tools > run lint.