Sorry for the poor examples.
These demonstrate the problem:
1. Original, presented as a gallery : https://tiagoferreira.nome.pt/photo/?m=20120813&lang=en
2. Translated, set as gallery but not presented as gallery: https://tiagoferreira.nome.pt/photo/?m=20120813&lang=pt
HI believe this is related with polylang, as it is poly lang which sets gallery for both posts all the time.
