Just as localization should begin early in the product cycle, so should testing of localized programs. It's very tempting to concentrate all testing efforts on the domestic product in order to get it shipped quickly. Early testing of all editions, however, ensures that development is paying attention to international issues because it often uncovers serious bugs in the core code.
When deciding whether to fix a bug, determine how much time fixing it will require and how crucial the fix is to the finished product. Does the bug make the program unusable? If not, will it seriously affect sales in any important target markets? Maintain common standards for all language editions of your product. Never postpone fixing bugs found in your domestic software product that will affect international users.
At the end of the product cycle, when the domestic product is close to shipping, it is tempting to postpone fixing bugs that arise only in localized software until after the domestic product ships, out of fear of "destabilizing" the domestic edition. But every time you delay fixing an international-related bug, you are postponing all of your international releases. If your domestic product will ship in a few days, postponing a few bugs exclusive to localized editions is not a big deal. However, many experienced developers have worked on projects that were in ship mode for weeks, months, or even years. Under these circumstances, allowing international bugs to pile up is dangerous and leaves unresolved a significant number of changes that you will have to merge with the sources for the next product release.
If you plan to ship a single international executable, you really cannot postpone fixing bugs related to world-readiness. Developers of localized editions might need a few extra weeks after the domestic edition ships to make changes to resource files, Help files, or sample documents; but once your single executable is done, it should be done for all languages. Of course, this won't always happen, but striving for this ideal will definitely benefit the ship dates of your localized editions.