Hi,
You can add localized text via the XMPScript API. Use SetLocalizedText() on the alt-text property. For example:
xmp.setLocalizedText(XMPConst.NS_DC, "title", "en", "en-US", "SomeEnglish);
xmp.setLocalizedText(XMPConst.NS_DC, "title", "fr", "fr-FR", "SomeFrench");
If you always supply both the generic, fr, and specific, fr-FR, tags then you should have no problems and a new entry will be added to the array unless there is already an exact match - in which case you would modify the entry.
Hope that makes sense.