When a wiki's main language is not English, the main language for the labels will most likely also not be English. The language fallback does not take this into account, and anyone viewing pages in another language will not see labels unless the content language happens to already be one of the fallback languages for the interface language.

For example, https://data.biblissima.fr/ has the content language set to French (mw.config.get("wgContentLanguage"); returns "fr"). The pages are in French and the items have labels in French. Because of my browser settings, it gives me the interface in German.

If I go to https://data.biblissima.fr/w/Sp%C3%A9cial:Toutes_les_pages?namespace=120&uselang=de, the first page in the list is shown as "(Q1000)" with no label, because it only has a label in French, and French is not one of the fallback languages for German.

Screenshots of how it is shown and the labels for the item being shown:

Similarly, if I go to https://data.biblissima.fr/w/Item:Q283909?uselang=de, the value for the "Verfasser" statement is shown as "Q1003", because that item also only has a label in French.

Screenshots of how it is shown and the labels for the item being shown: