Merge new strings with translations into "custom" localizations

Per the comment in both the TMX and XLIFF versions of FilenamesToAddToCache, we "Prefer the custom localizations folder version if it exists, otherwise the installed language folder." The problem with this is that a user might supply some missing translations, creating a custom localization. Even if they share those translations back to the developers and they are incorporated into a later version of the program, they'll continue to use their custom version, unless they are instructed to delete their local custom file so the program can use the "official" versions. That not only means that newly added or improved versions of strings for existing IDs will get ignored but -- even worse -- new translations for new string IDs will get ignored! I could not understand why I wasn't able to see the newly added translated strings for new string IDs until I stepped through the code and saw that comment.
This is probably fairly low exposure since very little field-based translation is probably happening for languages that have official translations. But it's pretty confusing and annoying when it does occur.

Start by tracing the custom-versus-installed localization selection described in the TMX and XLIFF versions of FilenamesToAddToCache. Determine how official updates should be combined with user-supplied entries; done should include updated existing strings and newly added string IDs without requiring users to delete their custom localization.