[RTL UI] adds bidi.lua, bootstrap UI mirroring with RTL languages

Set default language (for Harfbuzz to pick up localized glyphs
in a font), default text direction, and UI element mirroring
depending on the UI language.

frontend/ui/language.lua

@@ -1,7 +1,5 @@
 -- high level wrapper module for gettext
 
-local InfoMessage = require("ui/widget/infomessage")
-local UIManager = require("ui/uimanager")
 local _ = require("gettext")
 
 local Language = {
@@ -47,13 +45,46 @@ local Language = {
         zh_TW = "中文（台灣)",
         ["zh_TW.Big5"] = "中文（台灣）（Big5）",
     },
+    -- Languages that are written RTL, and should have the UI mirrored.
+    -- Should match lang tags defined in harfbuzz/src/hb-ot-tag-table.hh.
+    -- https://meta.wikimedia.org/wiki/Template:List_of_language_names_ordered_by_code
+    -- Not included are those absent or commented out in hb-ot-tag-table.hh.
+    languages_rtl = {
+        ar  = true, -- Arabic
+        arz = true, -- Egyptian Arabic
+        ckb = true, -- Sorani (Central Kurdish)
+        dv  = true, -- Divehi
+        fa  = true, -- Persian
+        he  = true, -- Hebrew
+        ks  = true, -- Kashmiri
+        ku  = true, -- Kurdish
+        ps  = true, -- Pashto
+        sd  = true, -- Sindhi
+        ug  = true, -- Uyghur
+        ur  = true, -- Urdu
+        yi  = true, -- Yiddish
+    }
 }
 
 function Language:getLanguageName(lang_locale)
     return self.language_names[lang_locale] or lang_locale
 end
 
+function Language:isLanguageRTL(lang_locale)
+    if not lang_locale then
+        return false
+    end
+    local lang = lang_locale
+    local sep = lang:find("_")
+    if sep then
+        lang = lang:sub(1, sep-1)
+    end
+    return self.languages_rtl[lang] or false
+end
+
 function Language:changeLanguage(lang_locale)
+    local InfoMessage = require("ui/widget/infomessage")
+    local UIManager = require("ui/uimanager")
     _.changeLang(lang_locale)
     G_reader_settings:saveSetting("language", lang_locale)
     UIManager:show(InfoMessage:new{