Regional datetime formats fixes

[kadavr95]

Since some time (late 2025 - early 2026) Tandoor has started showing wrong datetime formats for regional variations of languages. It is just using plain language formats, e.g. "en" (which is the same as "en_us") for both "en_us" and "en_gb".

This PR targets this issue, trying to make it work correctly for regional variants.

Here is a comparison of datetime formats for different combinations of interface and browser languages, which I have tested myself:

It compares the formats of:

• some old commit of TandoorRecipes before changes in i18n
• current develop branch state
• JS format, which is produced by console.log(new Date('2011-12-13T14:15').toLocaleString('en_gb'));
• branch which I currently propose to merge (Disclaimer: it was made using AI, based on my branch)
• my own efforts to tackle this problem in the https://github.com/kadavr95/recipes/tree/develop-manual branch. But theimplementation here was bad,as to fix the locale in date selectors I had just rolled back some changes made in TandoorRecipes during that i18n rework which led to current problems.

I guess this PR should close #4360, as it seems to be about this exact problem.

Disclaimer once again: the code changes were made with using AI

[vabene1111]

thank you and thank you for being open about using AI. I typically reject AI PRs but I will take a look when I have time as the issue does need to be fixed. I will see if I will merge or create a fix myself, depending on the origin of the issue.

[kadavr95]

You are welcome and thank you for creating and maintaining Tandoor. I disclose use of AI, because I know it is normally discouraged. Even though I want this problem fixed, I don't want to introduce new ones, due to my lack of TS+Vue experience. Take your time and feel free to reject my PR if needed, my goal is just to save my eyes from MM/DD/YYYY format. For now, I can use my own fork with either of my two solutions branches, but it would be great to not need it anymore. At least now I am sure that this problem along with some examples of when the current app is not working correctly is on the radar. Because my messages about it on Discord seemed to be missed and almost nobody has raised concerns about it in the issues here (to my surprise, not gonna lie) in more than half a year.

[vabene1111]

thank you, will review and see what I will do. Sorry to some unrepsonsiveness, time is extremly limited for me