Untranslated strings are skipped when site_locale is set
It is of course in translators' best interest to get everything translated, but it happens frequently that some translations are incomplete, be that due to a recent change in source texts or a lack of interest. Regardless, missing translations should not delete page contents for users who prefer reading app descriptions in their own language.
See, for instance, https://kde.org/applications/utilities/org.kde.filelight?site_locale=ru. The description contains only one line, which is the only line that is translated here: https://websvn.kde.org/trunk/l10n-kf5/ru/messages/kdeutils/org.kde.filelight.appdata.po?view=markup
Now let's take a look at https://kde.org/applications/development/org.kde.kdevelop?site_locale=ru. It has the English source text, likely because https://websvn.kde.org/trunk/l10n-kf5/ru/messages/extragear-kdevelop/org.kde.kdevelop.appdata.po?view=markup is virtually empty (only has a string for the name).
Finally, here's the page for Dolphin: https://kde.org/applications/system/org.kde.dolphin?site_locale=ru It appears complete because the translation is also complete: https://websvn.kde.org/trunk/l10n-kf5/ru/messages/applications/org.kde.dolphin.appdata.po?view=markup