This is a second attempt to localiseSymfony\Component\Form\Extension\Core\Type\TimezoneType. First attempt was#17628 that was rejected, because it changed the existing UI a bit, and because I was not fully aware of how to use the CLDR data.

I really think localisation is important. Not everyone speaks English, and if you visit e.g. a Russian-language site, you would expect the timezone picker to show city names written in Cyrillic letters. Let's find a way to fix this, even if it requires adjusting the existing UI slightly.

In this attempt I have tried to stay closer to the existing UI than in#17628. One intentional change, though, is that “Argentina - San Juan” is now displayed as “San Juan, Argentina”. CLDR uses this formatting for US states, e.g. “Tell City, Indiana”, and I think it makes sense, so that list items can be sorted alphabetically by the city name.

One improvent over the existing solution is thatAntarctica/DumontDUrville is now displayed as “Dumont d’Urville” rather than “DumontDUrville”.

Matching the current UI is a bit tricky, because CLDR does not contain translations for the oceans (Pacific/Atlantic/Indian), so they must be provided manually. But this is necessary, if we want to preserve the existing UI. One way to get around this (and simplify the code a bit) is to group timezones by continent as shown inthis list, rather than by the tzdata area. This will give nearly identical results, except that timezones inAtlantic/xxx,Pacific/xxx andIndian/xxx will be shown under the continent they belong to, and a few other exceptions (e.g. Turkey now belongs to Asia, while tzdata puts it in Europe, because Istanbil is there – seehttps://unicode.org/pipermail/cldr-users/2018-April/000795.html). These are all minor changes IMO.

I have chosen to base the implementation on the excellent Punic library rather than downloading the XML files directly, like I did in#17628, or trying to add them to the Intl component, like I did in#17636. The Punic library supports large parts of CLDR (the data and the specification on how to use it), so I think it is a good choice. It is not a runtime dependency, but is only used for generating the translation files, so I hope adding this dependency is not too controversial.

Disclaimer: I was recentlyinvited to become a co-author of Punic, because I had contributed a bit to the library. However, I do not have any interests in this library, except as an interested user and contributor.

One thing still missing from this PR is that timezones are not sorted according to their localised name. Stack Overflow tells me that in order to do that, one should translate them and sort them prior to adding them to the form. This is e.g. whenCountryType does. However, I'm not sure about the best way to do this (i.e. how to inject the translator). Another option is to use Punic as a runtime dependency to get the localised strings without using the translator, but I don't know if Symfony wants this kind of dependencies. It could be made as an optional dependency with fallback to the English names like to today. So I kindly ask for advice on how to handle this.