luci-base: fix luci.i18n.setlanguage()

Rework the setlanguage() implementation to actually switch catalogues
if another language has been loaded previously and change it to return
the effectively loaded language tag.

Also improve input parameter validation and accept tags in both lower
or upper case.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/i18n.lua

@@ -23,15 +23,31 @@ function loadc(file, force)
 end
 
 function setlanguage(lang)
-	context.lang   = lang:gsub("_", "-")
-	context.parent = (context.lang:match("^([a-z][a-z])_"))
-	if not tparser.load_catalog(context.lang, i18ndir) then
-		if context.parent then
-			tparser.load_catalog(context.parent, i18ndir)
+	local code, subcode = lang:match("^([A-Za-z][A-Za-z])[%-_]([A-Za-z][A-Za-z])$")
+	if not (code and subcode) then
+		subcode = lang:match("^([A-Za-z][A-Za-z])$")
+		if not subcode then
+			return nil
+		end
+	end
+
+	context.parent = code and code:lower()
+	context.lang   = context.parent and context.parent.."-"..subcode:lower() or subcode:lower()
+
+	if tparser.load_catalog(context.lang, i18ndir) and
+	   tparser.change_catalog(context.lang)
+	then
+		return context.lang
+
+	elseif context.parent then
+		if tparser.load_catalog(context.parent, i18ndir) and
+		   tparser.change_catalog(context.parent)
+		then
 			return context.parent
 		end
 	end
-	return context.lang
+
+	return nil
 end
 
 function translate(key)

modules/luci-base/luasrc/i18n.luadoc

@@ -37,7 +37,8 @@ Set the context default translation language.
 
 @class function
 @name setlanguage
-@param lang	Two-letter language code
+@param lang	An IETF/BCP 47 language tag or ISO3166 country code, e.g. "en-US" or "de"
+@return		The effective loaded language, e.g. "en" for "en-US" - or nil on failure
 ]]
 
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