Translation improvements: CLDR Plurals + Translatables

[zeripath]

This PR will provide the framework for a few translation improvements.

TranslatableStringer and TranslatableFormatted

The first thing is the ability to pass in translatable elements - this would allow us (for example) to create an error:

type TranslatableError struct {
Key string
Args []interface{}
}

func (err *TranslatableError) Error() string {
   return i18n.Tr("en-US", err.Key, err.Args...)
}

func (err *TranslatableError) TranslatedString(l i18n.Locale) {
   return l.Tr(err.Key, err.Args...)
}

TrPlural and TrOrdinal

The second part is the creation of TrPlural and TrOrdinal which use the CLDR data to correctly determine plurals for the locales. This is used to replace TrN.

Plural and ordinal keys can be written as go templates if the key has the suffix _plural or _ordinal:

example_plural=common text {{if .One}}Singular{{else}}Other{{end}}

These are interpreted at the time of locale reading and split into the appropriate forms for the language. Alternatively the individual plural forms can be provided separately as example_plural_one etc.

As the templates are parsed once at locale load time the magic suffix is required - I don't think this is too clunky. It would be possible to make it so we parse the template at time of TrPlural call but that would require the locale storing a text.Template or other locking mechanism, and I think the magic suffix isn't too inconvenient.

(TrN has been kept in place to help handle merges and old data.)

Fixes #23797

[codecov-commenter]

Codecov Report

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The diff coverage is 87.03%.

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Impacted Files	Coverage Δ
modules/test/context_tests.go	62.96% <0.00%> (ø)
modules/translation/i18n/i18n.go	77.77% <0.00%> (ø)
modules/translation/i18n/translatable.go	0.00% <0.00%> (ø)
modules/translation/translation.go	36.29% <ø> (ø)
routers/web/repo/migrate.go	38.80% <0.00%> (ø)
routers/web/repo/repo.go	20.00% <0.00%> (ø)
routers/web/repo/setting.go	16.71% <0.00%> (ø)
modules/translation/i18n/plurals/form.go	16.66% <16.66%> (ø)
modules/translation/i18n/format.go	40.62% <42.85%> (ø)
modules/translation/i18n/locale.go	44.04% <44.04%> (ø)
... and 4 more

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[zeripath]

Conflicts resolved.

[lafriks]

How that would work with crowdin? When English have two forms but other languages could have 3 or 4?

[zeripath]

example_plural = {{if .Zero}}zero{{else if .One}}one{{else}}other{{end}}form

Or add each form individually as example_plural_zero etc. (But I don't think crowdin will let you do that at present.)

Locale keys ending in _plural or _ordinal are parsed as go templates once at time of locale reading and the plural forms are extracted as per the forms within that language.

[zeripath]

@lafriks I've updated the description.

[wxiaoguang]

Generally L-G-T-M.

Could it be better if the plurals and the common/dtd are put in a new public go package? They seem quite complex and independent. Making them a new package would keep Gitea repo more focused. (and maybe other projects could re-use them)

[zeripath]

Generally L-G-T-M.

Could it be better if the plurals and the common/dtd are put in a new public go package? They seem quite complex and independent. Making them a new package would keep Gitea repo more focused. (and maybe other projects could re-use them)

I think we should try to figure out what works before export things. for example we need to include date and time formatting at some point.

[wxiaoguang]

We need ICU message system #23863, this homemade syntax is impossible to maintain.