[feature] Set/show instance language(s); show post language on frontend (#2362)

* update go text, include text/display

* [feature] Set instance langs, show post lang on frontend

* go fmt

* WebGet

* set language for whole article, don't use FA icon

* mention instance languages + other optional config vars

* little tweak

* put languages in config properly

* warn log language parse

* change some naming around

* tidy up validate a bit

* lint

* rename LanguageTmpl in template

internal/language/language.go

@@ -0,0 +1,184 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package language
+
+import (
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"golang.org/x/text/language"
+	"golang.org/x/text/language/display"
+)
+
+var namer display.Namer
+
+// InitLangs parses languages from the
+// given slice of tags, and sets the `namer`
+// display.Namer for the instance.
+//
+// This function should only be called once,
+// since setting the namer is not thread safe.
+func InitLangs(tagStrs []string) (Languages, error) {
+	var (
+		languages = make(Languages, len(tagStrs))
+		tags      = make([]language.Tag, len(tagStrs))
+	)
+
+	// Reset namer.
+	namer = nil
+
+	// Parse all tags first.
+	for i, tagStr := range tagStrs {
+		tag, err := language.Parse(tagStr)
+		if err != nil {
+			return nil, gtserror.Newf(
+				"error parsing %s as BCP47 language tag: %w",
+				tagStr, err,
+			)
+		}
+		tags[i] = tag
+	}
+
+	// Check if we can set a namer.
+	if len(tags) != 0 {
+		namer = display.Languages(tags[0])
+	}
+
+	// Fall namer back to English.
+	if namer == nil {
+		namer = display.Languages(language.English)
+	}
+
+	// Parse nice language models from tags
+	// (this will use the namer we just set).
+	for i, tag := range tags {
+		languages[i] = ParseTag(tag)
+	}
+
+	return languages, nil
+}
+
+// Language models a BCP47 language tag
+// along with helper strings for the tag.
+type Language struct {
+	// BCP47 language tag
+	Tag language.Tag
+	// Normalized string
+	// of BCP47 tag.
+	TagStr string
+	// Human-readable
+	// language name(s).
+	DisplayStr string
+}
+
+// MarshalText implements encoding.TextMarshaler{}.
+func (l *Language) MarshalText() ([]byte, error) {
+	return []byte(l.TagStr), nil
+}
+
+// UnmarshalText implements encoding.TextUnmarshaler{}.
+func (l *Language) UnmarshalText(text []byte) error {
+	lang, err := Parse(string(text))
+	if err != nil {
+		return err
+	}
+
+	*l = *lang
+	return nil
+}
+
+type Languages []*Language
+
+func (l Languages) Tags() []language.Tag {
+	tags := make([]language.Tag, len(l))
+	for i, lang := range l {
+		tags[i] = lang.Tag
+	}
+
+	return tags
+}
+
+func (l Languages) TagStrs() []string {
+	tagStrs := make([]string, len(l))
+	for i, lang := range l {
+		tagStrs[i] = lang.TagStr
+	}
+
+	return tagStrs
+}
+
+func (l Languages) DisplayStrs() []string {
+	displayStrs := make([]string, len(l))
+	for i, lang := range l {
+		displayStrs[i] = lang.DisplayStr
+	}
+
+	return displayStrs
+}
+
+// ParseTag parses and nicely formats the input language BCP47 tag,
+// returning a Language with ready-to-use display and tag strings.
+func ParseTag(tag language.Tag) *Language {
+	l := new(Language)
+	l.Tag = tag
+	l.TagStr = tag.String()
+
+	var (
+		// Our name for the language.
+		name string
+		// Language's name for itself.
+		selfName = display.Self.Name(tag)
+	)
+
+	// Try to use namer
+	// (if initialized).
+	if namer != nil {
+		name = namer.Name(tag)
+	}
+
+	switch {
+	case name == "":
+		// We don't have a name for
+		// this language, just use
+		// its own name for itself.
+		l.DisplayStr = selfName
+
+	case name == selfName:
+		// Avoid repeating ourselves:
+		// showing "English (English)"
+		// is not useful.
+		l.DisplayStr = name
+
+	default:
+		// Include our name for the
+		// language, and its own
+		// name for itself.
+		l.DisplayStr = name + " " + "(" + selfName + ")"
+	}
+
+	return l
+}
+
+// Parse parses and nicely formats the input language BCP47 tag,
+// returning a Language with ready-to-use display and tag strings.
+func Parse(lang string) (*Language, error) {
+	tag, err := language.Parse(lang)
+	if err != nil {
+		return nil, err
+	}
+
+	return ParseTag(tag), nil
+}