At some point, the design idea was that there is one [primary] title, which is a LocalizedString and will be displayed on the UI first and foremost.
Additionally, there can be any number of alternative titles (array of LocalizedString) that give additional, lower-priority titles to the manga.
A LocalizedString is a map of {locale: title}, where any number of unique(!) locale and title rows can be contained.
a primary title can be
Code:
{en: "5Toubun no Hanayome"}
with altTitles being
Code:
[
{en: "The Five Wedded Brides"},
{en: "The Five-Part Bride"},
{en: "The Quintessential Quintuplets"}
]
BUT, as you can see
from the request for that manga itself, languages that dont exist in the primary title tend to be put into alt titles, where by the original design idea the first (primary) title for each language should always be put into the title field first and altTitles second. It never really worked out like that, mostly because we already had a giant backlog of v3 imported titles that couldn't be converted and there was no easy way to force people to use it this way without dumb side effects.
Might be changed in the future