[BeckyEbook]

You are right.
A simple workaround would be an additional condition in two files:
Misc/GuideItems.cpp [inside GuideItems::GetTitle]
Misc/Landmarks.cpp [inside Landmarks::GetTitle]

Spoiler:

    // if no translator matched for xx-YY
    // try again with just part of language code that exists before the "-"
    if (!translation_loaded) {
        const QString langcut = lang.split("-").at(0);
        const QString qm_name = QString("sigil_%1").arg(langcut);
        // Run though all locations and stop once we find and are able to load
        // an appropriate translation.
        foreach(QString path, UILanguage::GetPossibleTranslationPaths()) {
            if (QDir(path).exists()) {
                if (bookTranslator.load(qm_name, path)) {
                    translation_loaded = true;
                    break;
               }
            }
        }
    }

Probably KevinH or DiapDealer will take a look and decide whether such a code is suitable.