diff --git a/src/env/__init_tls.c b/src/env/__init_tls.c
index 13cf2ee..efa0728 100644
--- a/src/env/__init_tls.c
+++ b/src/env/__init_tls.c
@@ -16,6 +16,7 @@ int __init_tp(void *p)
 	if (!r) libc.can_do_threads = 1;
 	libc.has_thread_pointer = 1;
 	td->tid = td->pid = __syscall(SYS_set_tid_address, &td->tid);
+	td->locale = &libc.global_locale;
 	return 0;
 }
 
diff --git a/src/internal/libc.h b/src/internal/libc.h
index fb4d9bc..037d16b 100644
--- a/src/internal/libc.h
+++ b/src/internal/libc.h
@@ -5,6 +5,11 @@
 #include <stdio.h>
 #include <limits.h>
 
+struct __locale_struct {
+	int ctype_utf8;
+	char *messages_name;
+};
+
 struct __libc {
 	int has_thread_pointer;
 	int can_do_threads;
@@ -16,6 +21,9 @@ struct __libc {
 	int ofl_lock[2];
 	size_t tls_size;
 	size_t page_size;
+	volatile int uselocale_cnt;
+	volatile int bytelocale_cnt_minus_1;
+	struct __locale_struct global_locale;
 };
 
 extern size_t __hwcap;
diff --git a/src/internal/locale_impl.h b/src/internal/locale_impl.h
index f41c6f2..2747b85 100644
--- a/src/internal/locale_impl.h
+++ b/src/internal/locale_impl.h
@@ -1,5 +1,17 @@
 #include <locale.h>
+#include <stdlib.h>
+#include "libc.h"
+#include "pthread_impl.h"
 
-struct __locale_struct {
-	int dummy;
-};
+#define LOCALE_NAME_MAX 15
+
+int __setlocalecat(locale_t, int, const char *);
+
+#define CURRENT_LOCALE \
+	(libc.uselocale_cnt ? __pthread_self()->locale : &libc.global_locale)
+
+#define CURRENT_UTF8 \
+	(libc.bytelocale_cnt_minus_1<0 || __pthread_self()->locale->ctype_utf8)
+
+#undef MB_CUR_MAX
+#define MB_CUR_MAX (CURRENT_UTF8 ? 4 : 1)
diff --git a/src/locale/__setlocalecat.c b/src/locale/__setlocalecat.c
new file mode 100644
index 0000000..f1e4bf0
--- /dev/null
+++ b/src/locale/__setlocalecat.c
@@ -0,0 +1,46 @@
+#include <locale.h>
+#include <string.h>
+#include "locale_impl.h"
+#include "libc.h"
+#include "atomic.h"
+
+static const char envvars[][12] = {
+	"LC_CTYPE",
+	"LC_NUMERIC",
+	"LC_TIME",
+	"LC_COLLATE",
+	"LC_MONETARY",
+	"LC_MESSAGES",
+};
+
+int __setlocalecat(locale_t loc, int cat, const char *val)
+{
+	if (!*val) {
+		(val = getenv("LC_ALL")) ||
+		(val = getenv(envvars[cat])) ||
+		(val = getenv("LANG")) ||
+		(val = "C.UTF-8");
+	}
+
+	size_t n = strnlen(val, LOCALE_NAME_MAX);
+	int builtin = (val[0]=='C' && !val[1])
+		|| !strcmp(val, "C.UTF-8")
+		|| !strcmp(val, "POSIX");
+
+	switch (cat) {
+	case LC_CTYPE:
+		a_store(&loc->ctype_utf8, !builtin || val[1]=='.');
+		break;
+	case LC_MESSAGES:
+		if (builtin) {
+			loc->messages_name[0] = 0;
+		} else {
+			memcpy(loc->messages_name, val, n);
+			loc->messages_name[n] = 0;
+		}
+		/* fall through */
+	default:
+		break;
+	}
+	return 0;
+}
diff --git a/src/locale/duplocale.c b/src/locale/duplocale.c
index f9fc1ff..1336870 100644
--- a/src/locale/duplocale.c
+++ b/src/locale/duplocale.c
@@ -3,12 +3,19 @@
 #include "locale_impl.h"
 #include "libc.h"
 
-locale_t duplocale(locale_t old)
+locale_t __duplocale(locale_t old)
 {
-	locale_t new;
-	new = calloc(1, sizeof *new);
+	locale_t new = calloc(1, sizeof *new + LOCALE_NAME_MAX + 1);
+	if (!new) return 0;
+	new->messages_name = (void *)(new+1);
+
+	if (old == LC_GLOBAL_LOCALE) old = &libc.global_locale;
+	new->ctype_utf8 = old->ctype_utf8;
+	if (old->messages_name)
+		strcpy(new->messages_name, old->messages_name);
+
 	if (new && old != LC_GLOBAL_LOCALE) memcpy(new, old, sizeof *new);
 	return new;
 }
 
-weak_alias(duplocale, __duplocale);
+weak_alias(__duplocale, duplocale);
diff --git a/src/locale/langinfo.c b/src/locale/langinfo.c
index 7bb56ee..13abf45 100644
--- a/src/locale/langinfo.c
+++ b/src/locale/langinfo.c
@@ -1,5 +1,6 @@
 #include <locale.h>
 #include <langinfo.h>
+#include "locale_impl.h"
 #include "libc.h"
 
 static const char c_time[] =
@@ -60,7 +61,7 @@ char *__nl_langinfo_l(nl_item item, locale_t loc)
 
 char *__nl_langinfo(nl_item item)
 {
-	return __nl_langinfo_l(item, 0);
+	return __nl_langinfo_l(item, CURRENT_LOCALE);
 }
 
 weak_alias(__nl_langinfo, nl_langinfo);
diff --git a/src/locale/newlocale.c b/src/locale/newlocale.c
index 447c8fc..39501d0 100644
--- a/src/locale/newlocale.c
+++ b/src/locale/newlocale.c
@@ -3,12 +3,24 @@
 #include "locale_impl.h"
 #include "libc.h"
 
-locale_t newlocale(int mask, const char *name, locale_t base)
+locale_t __newlocale(int mask, const char *name, locale_t loc)
 {
-	if (*name && strcmp(name, "C") && strcmp(name, "POSIX"))
-		return 0;
-	if (!base) base = calloc(1, sizeof *base);
-	return base;
+	int i;
+
+	if (!loc) {
+		loc = calloc(1, sizeof *loc + LOCALE_NAME_MAX + 1);
+		if (!loc) return 0;
+		loc->messages_name = (void *)(loc+1);
+		for (i=0; i<LC_ALL; i++)
+			if (!(mask & (1<<i)))
+				__setlocalecat(loc, i, "");
+	}
+
+	for (i=0; i<LC_ALL; i++)
+		if (mask & (1<<i))
+			__setlocalecat(loc, i, name);
+
+	return loc;
 }
 
-weak_alias(newlocale, __newlocale);
+weak_alias(__newlocale, newlocale);
diff --git a/src/locale/setlocale.c b/src/locale/setlocale.c
index 28f29b8..cbc0b55 100644
--- a/src/locale/setlocale.c
+++ b/src/locale/setlocale.c
@@ -1,9 +1,67 @@
 #include <locale.h>
+#include <stdlib.h>
+#include <string.h>
+#include "locale_impl.h"
+#include "libc.h"
+#include "atomic.h"
 
-char *setlocale(int category, const char *locale)
+static char buf[2+4*(LOCALE_NAME_MAX+1)];
+
+char *setlocale(int cat, const char *name)
 {
-	/* Note: plain "C" would be better, but puts some broken
-	 * software into legacy 8-bit-codepage mode, ignoring
-	 * the standard library's multibyte encoding */
-	return "C.UTF-8";
+	if (!libc.global_locale.messages_name) {
+		libc.global_locale.messages_name =
+			buf + 2 + 3*(LOCALE_NAME_MAX+1);
+	}
+
+	if ((unsigned)cat > LC_ALL) return 0;
+
+	/* For LC_ALL, setlocale is required to return a string which
+	 * encodes the current setting for all categories. The format of
+	 * this string is unspecified, and only the following code, which
+	 * performs both the serialization and deserialization, depends
+	 * on the format, so it can easily be changed if needed. */
+	if (cat == LC_ALL) {
+		if (name) {
+			char part[LOCALE_NAME_MAX+1];
+			int i, j;
+			if (name[0] && name[1]==';'
+			    && strlen(name) > 2 + 3*(LOCALE_NAME_MAX+1)) {
+				part[0] = name[0];
+				part[1] = 0;
+				setlocale(LC_CTYPE, part);
+				part[LOCALE_NAME_MAX] = 0;
+				for (i=LC_TIME; i<LC_MESSAGES; i++) {
+					memcpy(part, name + 2 + (i-2)*(LOCALE_NAME_MAX+1), LOCALE_NAME_MAX);
+					for (j=LOCALE_NAME_MAX-1; j && part[j]==';'; j--)
+						part[j] = 0;
+					setlocale(i, part);
+				}
+				setlocale(LC_MESSAGES, name + 2 + 3*(LOCALE_NAME_MAX+1));
+			} else {
+				for (i=0; i<LC_ALL; i++)
+					setlocale(i, name);
+			}
+		}
+		memset(buf, ';', 2 + 3*(LOCALE_NAME_MAX+1));
+		buf[0] = libc.global_locale.ctype_utf8 ? 'U' : 'C';
+		return buf;
+	}
+
+	if (name) {
+		int adj = libc.global_locale.ctype_utf8;
+		__setlocalecat(&libc.global_locale, cat, name);
+		adj -= libc.global_locale.ctype_utf8;
+		if (adj) a_fetch_add(&libc.bytelocale_cnt_minus_1, adj);
+	}
+
+	switch (cat) {
+	case LC_CTYPE:
+		return libc.global_locale.ctype_utf8 ? "C.UTF-8" : "C";
+	case LC_MESSAGES:
+		return libc.global_locale.messages_name[0]
+			? libc.global_locale.messages_name : "C";
+	default:
+		return "C";
+	}
 }
diff --git a/src/locale/strfmon.c b/src/locale/strfmon.c
index e25db97..7cf2136 100644
--- a/src/locale/strfmon.c
+++ b/src/locale/strfmon.c
@@ -3,6 +3,7 @@
 #include <stdarg.h>
 #include <monetary.h>
 #include <errno.h>
+#include "locale_impl.h"
 
 static ssize_t vstrfmon_l(char *s, size_t n, locale_t loc, const char *fmt, va_list ap)
 {
@@ -93,7 +94,7 @@ ssize_t strfmon(char *restrict s, size_t n, const char *restrict fmt, ...)
 	ssize_t ret;
 
 	va_start(ap, fmt);
-	ret = vstrfmon_l(s, n, 0, fmt, ap);
+	ret = vstrfmon_l(s, n, CURRENT_LOCALE, fmt, ap);
 	va_end(ap);
 
 	return ret;
diff --git a/src/locale/uselocale.c b/src/locale/uselocale.c
index 4fc5c64..5106795 100644
--- a/src/locale/uselocale.c
+++ b/src/locale/uselocale.c
@@ -2,12 +2,25 @@
 #include "pthread_impl.h"
 #include "libc.h"
 
-locale_t uselocale(locale_t l)
+locale_t __uselocale(locale_t new)
 {
 	pthread_t self = __pthread_self();
 	locale_t old = self->locale;
-	if (l) self->locale = l;
-	return old;
+	locale_t global = &libc.global_locale;
+
+	if (new == LC_GLOBAL_LOCALE) new = global;
+
+	if (new && new != old) {
+		int adj = 0;
+		if (new == global) a_dec(&libc.uselocale_cnt);
+		else if (!new->ctype_utf8) adj++;
+		if (old == global) a_inc(&libc.uselocale_cnt);
+		else if (!old->ctype_utf8) adj--;
+		a_fetch_add(&libc.bytelocale_cnt_minus_1, adj);
+		self->locale = new;
+	}
+
+	return old == global ? LC_GLOBAL_LOCALE : old;
 }
 
-weak_alias(uselocale, __uselocale);
+weak_alias(__uselocale, uselocale);
diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index e9c8160..a7493c1 100644
--- a/src/thread/pthread_create.c
+++ b/src/thread/pthread_create.c
@@ -57,6 +57,12 @@ _Noreturn void pthread_exit(void *result)
 		exit(0);
 	}
 
+	if (self->locale != &libc.global_locale) {
+		a_dec(&libc.uselocale_cnt);
+		if (self->locale->ctype_utf8)
+			a_dec(&libc.bytelocale_cnt_minus_1);
+	}
+
 	if (self->detached && self->map_base) {
 		/* Detached threads must avoid the kernel clear_child_tid
 		 * feature, since the virtual address will have been
@@ -205,6 +211,7 @@ int pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict attrp
 	new->start_arg = arg;
 	new->self = new;
 	new->tsd = (void *)tsd;
+	new->locale = &libc.global_locale;
 	if (attr._a_detach) {
 		new->detached = 1;
 		flags -= CLONE_CHILD_CLEARTID;
diff --git a/src/time/strftime.c b/src/time/strftime.c
index 75ebca6..794fbe1 100644
--- a/src/time/strftime.c
+++ b/src/time/strftime.c
@@ -5,6 +5,7 @@
 #include <locale.h>
 #include <time.h>
 #include <limits.h>
+#include "locale_impl.h"
 #include "libc.h"
 #include "time_impl.h"
 
@@ -263,7 +264,7 @@ size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const st
 
 size_t strftime(char *restrict s, size_t n, const char *restrict f, const struct tm *restrict tm)
 {
-	return __strftime_l(s, n, f, tm, 0);
+	return __strftime_l(s, n, f, tm, CURRENT_LOCALE);
 }
 
 weak_alias(__strftime_l, strftime_l);
diff --git a/src/time/wcsftime.c b/src/time/wcsftime.c
index 8d2a2eb..638e64f 100644
--- a/src/time/wcsftime.c
+++ b/src/time/wcsftime.c
@@ -1,6 +1,7 @@
 #include <wchar.h>
 #include <time.h>
 #include <locale.h>
+#include "locale_impl.h"
 #include "libc.h"
 
 const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *tm, locale_t loc);
@@ -64,7 +65,7 @@ size_t __wcsftime_l(wchar_t *restrict s, size_t n, const wchar_t *restrict f, co
 
 size_t wcsftime(wchar_t *restrict wcs, size_t n, const wchar_t *restrict f, const struct tm *restrict tm)
 {
-	return __wcsftime_l(wcs, n, f, tm, 0);
+	return __wcsftime_l(wcs, n, f, tm, CURRENT_LOCALE);
 }
 
 weak_alias(__wcsftime_l, wcsftime_l);