Default language should be based on user babel box

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@Anthere Please can we have a detailed description of this task?

Indeed. I quickly entered that as it came from the suggestion of a third party. But it is not clear I see that now.
I think the concept is "when someone launches the tool, by default, the interface is in English. Then after the person logs in, the language of the interface should be based on the language preference used on Wikimedia Commons". Of course, by default, when no translation for that language is available on ISA, it will stay in English. But "user babel box" is "language preference on commons"