Translator: add "Translate from book language" option (#8094)

When enabled, if the book has some supported language tag
in its metadata, use it as the source language. Otherwise,
fallback to the current settings (auto-detect or selected
source language).

frontend/apps/reader/modules/readertypography.lua

@@ -113,6 +113,8 @@ for __, v in ipairs(LANGUAGES) do
         ReaderTypography.HYPH_DICT_NAME_TO_LANG_NAME_TAG[hyph_filename] = { lang_name, lang_tag }
     end
 end
+-- Make lang aliases available to other modules (can be used by Translator)
+ReaderTypography.LANG_ALIAS_TO_LANG_TAG = LANG_ALIAS_TO_LANG_TAG
 
 function ReaderTypography:init()
     self.menu_table = {}