diff --git a/IkiWiki.pm b/IkiWiki.pm
index 4964f97..19b70d0 100644
--- a/IkiWiki.pm
+++ b/IkiWiki.pm
@@ -42,6 +42,7 @@ sub defaultconfig () { #{{{
 	srcdir => undef,
 	destdir => undef,
 	pingurl => [],
+	lang => undef,
 	templatedir => "/usr/share/ikiwiki/templates",
 	underlaydir => "/usr/share/ikiwiki/basewiki",
 	setup => undef,
@@ -50,7 +51,40 @@ sub defaultconfig () { #{{{
 	plugin => [qw{mdwn inline htmlscrubber}],
 	timeformat => '%c',
 } #}}}
-   
+  
+sub _getlocalelist (;$) {
+	my $locale=shift;
+	if (! defined $locale || ! length $locale) {
+		return qw(en en_US C);
+	}
+	$locale=~s/(@[^.]+)//;
+	my ($lang, $territory, $charset)=($locale=~m/^
+	     ([^_@.]+)      #  Language
+	     (_[^_@.]+)?    #  Territory
+	     (\..+)?        #  Charset
+	     /x);
+	return defined $territory ? ("${lang}${territory}", $lang) : ($lang);
+}
+sub getlangs ($) {
+	my $lang=shift;
+	if (defined $lang && $lang eq 'auto') {
+		require POSIX;
+		$lang=POSIX::setlocale(5); # LC_MESSAGES
+	}
+	return _getlocalelist($lang);
+}
+
+sub langdir ($@) {
+	my $rootdir=shift;
+	my @langs=shift;
+	foreach my $try (@langs) {
+		my $maybe="${rootdir}/${try}";
+		return $maybe if -d $maybe;
+	}
+	debug("No matching language dir for '@langs'; falling back to '$rootdir'");
+	return $rootdir;
+}
+
 sub checkconfig () { #{{{
 	if ($config{w3mmode}) {
 		eval q{use Cwd q{abs_path}};
@@ -70,7 +104,15 @@ sub checkconfig () { #{{{
 	
 	$config{wikistatedir}="$config{srcdir}/.ikiwiki"
 		unless exists $config{wikistatedir};
-	
+
+	# Sanitize lang.
+	my @langs=getlangs($config{lang});
+	$config{lang}=$langs[0] unless exists $config{lang};
+
+	# Honor multilang directory structures.
+	$config{templatedir}=langdir($config{templatedir}, @langs);
+	$config{underlaydir}=langdir($config{underlaydir}, @langs);
+
 	if ($config{rcs}) {
 		eval qq{require IkiWiki::Rcs::$config{rcs}};
 		if ($@) {
diff --git a/IkiWiki/CGI.pm b/IkiWiki/CGI.pm
index 53e151d..292fa54 100644
--- a/IkiWiki/CGI.pm
+++ b/IkiWiki/CGI.pm
@@ -75,6 +75,7 @@ sub cgi_signin ($$) { #{{{
 		fields => [qw(do title page subpage from name password confirm_password email)],
 		header => 1,
 		charset => "utf-8",
+		messages => ":$config{lang}",
 		method => 'POST',
 		validate => {
 			confirm_password => {
@@ -241,6 +242,7 @@ sub cgi_prefs ($$) { #{{{
 		              subscriptions locked_pages)],
 		header => 0,
 		charset => "utf-8",
+		messages => ":$config{lang}",
 		method => 'POST',
 		validate => {
 			confirm_password => {
@@ -313,6 +315,7 @@ sub cgi_editpage ($$) { #{{{
 		fields => [qw(do rcsinfo subpage from page editcontent comments)],
 		header => 1,
 		charset => "utf-8",
+		messages => ":$config{lang}",
 		method => 'POST',
 		validate => {
 			editcontent => '/.+/',