Support for localization in PkPass files

[ueman]

Text can be localized, but images can be localized too.

PkPass docs for localizations (text and images):

• https://developer.apple.com/library/archive/documentation/UserExperience/Conceptual/PassKit_PG/Creating.html#//apple_ref/doc/uid/TP40012195-CH4-SW54
• https://developer.apple.com/documentation/walletpasses/creating_the_source_for_a_pass

Docs for .strings file format description:

• https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/LoadingResources/Strings/Strings.html

.strings files are UTF-16 not UTF-8, so the files have to be read like this StackOverflow post suggests. This issue dart-lang/core#266 contains some hints too.

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There are various approaches to parsing a .strings file:

• use a regex (I believe strings files for pkpasses technically can't have placeholders)
• write a custom parser using petiteparser
• naive custom string parsing

Personally, I would like to see a custom parser based on petiteparser with all the bells and whistles, but I think using a RegEx at first should be sufficient.

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The API could look like one of the following examples:

class PkPass {
  String getTranslated(String key, Locale locale) { ... }
}

// and then you can use it something like this:

PkPass pass = ...;
pass.getTranslated(pass.pass.description, locale);

With intl

pass.pass.description;
// provide a localized getter that makes use of Intl.defaultLocale or is alternatively set somehow
pass.pass.descriptionLocalized;

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This paragraph is only relevant for passkit_UI

Language fallback rules (should be applied on a per field basis):

1. No translations -> Nothing gets translated
2. The users' language gets chosen if available (e.g. de_DE)
   • Localizations.of(context).locale
3. The users' language without country code gets chosen if available (e.g. de instead of de_DE)
   • Localizations.of(context).locale
4. Otherwise, fallback to what's reported via PlatformDispatcher.locales (while skipping already tried languages)
   • First the languages are tested with country codes (de_DE), then without (just de)
5. If a translation is missing, fall back to the translation key

Formatted values like dates, numbers, etc are always localized to the users' current language (as described here)

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The default localized value should be https://api.dart.dev/stable/3.4.4/dart-io/Platform/localeName.html on dart:io and whatever browser equivalent there is. On Flutter it should use the Localization.of().locale as default.

[ueman]

This is now mostly usable, albeit not automatic