I got a shop with 4 different languages en/de/fr/it. Now I installed translation modules which have their files under app/i18n additionally I have a custom theme under with language files under app/design/frontend/mine/custom/i18n
all that seems to work fine if I use php. All translations are present. If I use js, the translations are not present. I checked the js-translation.json it regenerates during static file generation but does not include the translation strings. But it is also not empty. A fraction of translations is added. If I manually add my string for testing then it's being translated in js. So it's just not added for some reason.
Does someone know why or have a hint how I can track that issue down?