feat(i18n): Add i18n support for as many languages as possible

(Cherry picked from 556103e but content mixed with later commits from
rebased i18n-for-all branch.)

For fetching a key from the rebased source  branch:

    function copykey () {
        sed -i -e "/^$1 = /d" *.ftl
        for ftl in *.ftl; do
            rg "^$1 =" ../i18n-for-all/i18n/$ftl >> $ftl
        done
    }

classes/book.lua

@@ -104,30 +104,16 @@ function class:registerCommands ()
     if SU.boolean(options.toc, true) then
       SILE.call("tocentry", { level = level, number = number }, SU.subContent(content))
     end
-    local lang = SILE.settings:get("document.language")
     if SU.boolean(options.numbering, true) then
-      if options.prenumber then
-        if SILE.Commands[options.prenumber .. ":"  .. lang] then
-          options.prenumber = options.prenumber .. ":" .. lang
-        end
-        SILE.call(options.prenumber)
-      end
-      SILE.call("show-multilevel-counter", { id = "sectioning" })
-      if options.postnumber then
-        if SILE.Commands[options.postnumber .. ":" .. lang] then
-          options.postnumber = options.postnumber .. ":" .. lang
-        end
-        SILE.call(options.postnumber)
+      if options.msg then
+        SILE.call("fluent", { number = number }, { options.msg })
+      else
+        SILE.call("show-multilevel-counter", { id = "sectioning" })
       end
     end
   end)
 
-  self:registerCommand("book:chapter:pre", function (_, _)
-    SILE.call("fluent", {}, { "book-chapter-title-pre" })
-  end)
-
   self:registerCommand("book:chapter:post", function (_, _)
-    SILE.call("fluent", {}, { "book-chapter-post" })
     SILE.call("par")
   end)
 
@@ -157,10 +143,15 @@ function class:registerCommands ()
         numbering = options.numbering,
         toc = options.toc,
         level = 1,
-        prenumber = "book:chapter:pre",
-        postnumber = "book:chapter:post"
+        msg = "book-chapter-title"
       }, content)
     end)
+    local lang = SILE.settings:get("document.language")
+    local postcmd = "book:chapter:post"
+    if SILE.Commands[postcmd .. ":" .. lang] then
+      postcmd = postcmd .. ":" .. lang
+    end
+    SILE.call(postcmd)
     SILE.call("book:chapterfont", {}, content)
     SILE.call("left-running-head", {}, function ()
       SILE.settings:temporarily(function ()
@@ -180,9 +171,14 @@ function class:registerCommands ()
       SILE.call("book:sectioning", {
         numbering = options.numbering,
         toc = options.toc,
-        level = 2,
-        postnumber = "book:section:post"
+        level = 2
       }, content)
+      local lang = SILE.settings:get("document.language")
+      local postcmd = "book:section:post"
+      if SILE.Commands[postcmd .. ":" .. lang] then
+        postcmd = postcmd .. ":" .. lang
+      end
+      SILE.call(postcmd)
       SILE.process(content)
     end)
     if not SILE.scratch.counters.folio.off then
@@ -215,11 +211,16 @@ function class:registerCommands ()
       SILE.call("book:sectioning", {
             numbering = options.numbering,
             toc = options.toc,
-            level = 3,
-            postnumber = "book:subsection:post"
+            level = 3
           }, content)
       SILE.process(content)
     end)
+    local lang = SILE.settings:get("document.language")
+    local postcmd = "book:subsection:post"
+    if SILE.Commands[postcmd .. ":" .. lang] then
+      postcmd = postcmd .. ":" .. lang
+    end
+    SILE.call(postcmd)
     SILE.typesetter:leaveHmode()
     SILE.call("novbreak")
     SILE.call("medskip")

i18n/af.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Hoofstuk { $number }
+
+tableofcontents-title = Inhoudsopgawe

i18n/am.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = ክፍል { $number }
+
+tableofcontents-title = ይዘት

i18n/ar.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = باب { $number }
+
+tableofcontents-title = المحتويات

i18n/as.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = অধ্যায় { $number }
+
+tableofcontents-title = সমল

i18n/bg.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Глава { $number }
+
+tableofcontents-title = Съдържание

i18n/bn.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = অধ্যায় { $number }
+
+tableofcontents-title = সূচীপত্র

i18n/bo.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = ལེའུ་ { $number }
+
+tableofcontents-title = དཀར་ཆག།

i18n/ca.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Capítol { $number }
+
+tableofcontents-title = Índex

i18n/cs.ftl

@@ -1,3 +1,9 @@
-book-chapter-title-pre = Kapitola{" "}
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Kapitola { $number }
 
 tableofcontents-title = Obsah

i18n/cy.ftl

@@ -0,0 +1,3 @@
+book-chapter-title = Pennod { $number }
+
+tableofcontents-title = Cynnwys

i18n/da.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Kapitel { $number }
+
+tableofcontents-title = Indhold

i18n/de.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Kapitel { $number }
+
+tableofcontents-title = Inhaltsverzeichnis

i18n/el-monoton.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Κεφάλαιο { $number }
+
+tableofcontents-title = Περιεχόμενα

i18n/el-polyton.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Κεφάλαιο { $number }
+
+tableofcontents-title = Περιεχόμενα

i18n/el.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Κεφάλαιο { $number }
+
+tableofcontents-title = Περιεχόμενα

i18n/en.ftl

@@ -6,7 +6,9 @@ bibliography-et-al = et al.
 
 bibliography-translated-by = Translated by { $name }
 
-book-chapter-title-pre = Chapter{" "}
+book-chapter-title = { chapter } { $number }
+
+book-chapter-title = Chapter { $number }
 
 hello = Hello <em>{ $name }</em>!
 

i18n/eo.ftl

@@ -1,12 +1,18 @@
 bibliography-and = kaj
 
+bibliography-edited-by = { $name }
+
 bibliography-edited-by = Redaktita por { $name }
 
+bibliography-et-al = et al.
+
 bibliography-et-al = kaj aliaj
 
+bibliography-translated-by = { $name }
+
 bibliography-translated-by = Tradukita por { $name }
 
-book-chapter-title-pre = Ĉapitro{" "}
+book-chapter-title = Ĉapitro { $number }
 
 hello = Saluton <em>{ $name }</em>!
 

i18n/es.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Capítulo { $number }
+
+tableofcontents-title = Índice

i18n/et.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Peatükk { $number }
+
+tableofcontents-title = Sisukord

i18n/eu.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title ={ $number }. Kapitulua
+
+tableofcontents-title = Gaien Aurkibidea

i18n/fi.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Luku { $number }
+
+tableofcontents-title = Sisällys

i18n/fr.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = Édité par { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = Traduit par { $name }
+
+book-chapter-title = Chapitre { $number }
+
+tableofcontents-title = Table des matières

i18n/ga.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Caibidil { $number }
+
+tableofcontents-title = Clár Ábhair

i18n/gu.ftl

@@ -0,0 +1,5 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }

i18n/hi.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = अध्याय { $number }
+
+tableofcontents-title = अनुक्रम

i18n/hr.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Poglavlje { $number }
+
+tableofcontents-title = Sadržaj

i18n/hu.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = { number }. fejezet
+
+tableofcontents-title = Tartalomjegyzék

i18n/hy.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Գլուխ { $number }
+
+tableofcontents-title = Բովանդակություն

i18n/id.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Bab { $number }
+
+tableofcontents-title = Daftar Isi

i18n/is.ftl

@@ -0,0 +1,9 @@
+bibliography-edited-by = { $name }
+
+bibliography-et-al = et al.
+
+bibliography-translated-by = { $name }
+
+book-chapter-title = Kafli { $number }
+
+tableofcontents-title = Efnisyfirlit