Not as a solution, because I don't know the exact setup you have, but as some background info:
Since a few updates ago, date format seems to be more closely linked to the chosen UI language.
Then when TB language is OS language, and only then it seems, TB next looks at OS settings.
Meaning: when you use TB in English and OS is English, then on a Canadian, US and UK system, TB's output would be YMD, MDY, and DMY respectively.
Similarly if you'd use French and OS is French with local settings, Canada would give YMD (presuming they use it in French as well) and France would give DMY.
But, my hunch is, you may be having a problem when you set TB to let's say German, Italian or Russian while on a OS that is not that language, more pragmatically a non-European set OS. I haven't tested extensively, but it could be that TB says: OK this is not the OS language, so I am not going to look at OS either, but follow TB's default lead.
This is as far as the UI is concerned. The template output would follow these settings, but in most of the date fields, the format can be hardcoded. You may e.g. have seen DD.MM in the past, where that dot is typical for only a few countries. This was driven by the html coding within the template, which rather unnecessarily hardformatted that particular date field. In your particular setup a hardformatted date field may be useful though.