Index: head/lib/libc/stdlib/strfmon.c
===================================================================
--- head/lib/libc/stdlib/strfmon.c
+++ head/lib/libc/stdlib/strfmon.c
@@ -92,7 +92,8 @@
 } while (0)
 
 #define GRPSEP do {						\
-	*--bufend = thousands_sep;				\
+	bufend -= thousands_sep_size;				\
+	memcpy(bufend, thousands_sep, thousands_sep_size);	\
 	groups++;						\
 } while (0)
 
@@ -520,7 +521,7 @@
 	return (chars);
 }
 
-/* convert double to ASCII */
+/* convert double to locale-encoded string */
 static char *
 __format_grouped_double(double value, int *flags,
 			int left_prec, int right_prec, int pad_char) {
@@ -536,19 +537,24 @@
 
 	struct lconv	*lc = localeconv();
 	char		*grouping;
-	char		decimal_point;
-	char		thousands_sep;
+	const char	*decimal_point;
+	const char	*thousands_sep;
+	size_t		decimal_point_size;
+	size_t		thousands_sep_size;
 
 	int groups = 0;
 
 	grouping = lc->mon_grouping;
-	decimal_point = *lc->mon_decimal_point;
-	if (decimal_point == '\0')
-		decimal_point = *lc->decimal_point;
-	thousands_sep = *lc->mon_thousands_sep;
-	if (thousands_sep == '\0')
-		thousands_sep = *lc->thousands_sep;
+	decimal_point = lc->mon_decimal_point;
+	if (*decimal_point == '\0')
+		decimal_point = lc->decimal_point;
+	thousands_sep = lc->mon_thousands_sep;
+	if (*thousands_sep == '\0')
+		thousands_sep = lc->thousands_sep;
 
+	decimal_point_size = strlen(decimal_point);
+	thousands_sep_size = strlen(thousands_sep);
+
 	/* fill left_prec with default value */
 	if (left_prec == -1)
 		left_prec = 0;
@@ -574,7 +580,8 @@
 		return (NULL);
 
 	/* make sure that we've enough space for result string */
-	bufsize = avalue_size * 2 + 1;
+	bufsize = avalue_size * (1 + thousands_sep_size) + decimal_point_size +
+	    1;
 	rslt = calloc(1, bufsize);
 	if (rslt == NULL) {
 		free(avalue);
@@ -593,12 +600,13 @@
 		bufend -= right_prec;
 		memcpy(bufend, avalue + avalue_size+padded-right_prec,
 		    right_prec);
-		*--bufend = decimal_point;
+		bufend -= decimal_point_size;
+		memcpy(bufend, decimal_point, decimal_point_size);
 		avalue_size -= (right_prec + 1);
 	}
 
 	if ((*flags & NEED_GROUPING) &&
-	    thousands_sep != '\0' &&	/* XXX: need investigation */
+	    thousands_sep_size > 0 &&	/* XXX: need investigation */
 	    *grouping != CHAR_MAX &&
 	    *grouping > 0) {
 		while (avalue_size > (int)*grouping) {
@@ -626,8 +634,9 @@
 	} else {
 		bufend -= avalue_size;
 		memcpy(bufend, avalue+padded, avalue_size);
+		/* decrease assumed $decimal_point */
 		if (right_prec == 0)
-			padded--;	/* decrease assumed $decimal_point */
+			padded -= decimal_point_size;
 	}
 
 	/* do padding with pad_char */
Index: head/lib/libc/tests/stdlib/Makefile
===================================================================
--- head/lib/libc/tests/stdlib/Makefile
+++ head/lib/libc/tests/stdlib/Makefile
@@ -6,6 +6,7 @@
 ATF_TESTS_C+=		mergesort_test
 ATF_TESTS_C+=		qsort_test
 ATF_TESTS_C+=		set_constraint_handler_s_test
+ATF_TESTS_C+=		strfmon_test
 ATF_TESTS_C+=		tsearch_test
 .if ${COMPILER_FEATURES:Mc++11}
 ATF_TESTS_CXX+=		cxa_thread_atexit_test
Index: head/lib/libc/tests/stdlib/strfmon_test.c
===================================================================
--- head/lib/libc/tests/stdlib/strfmon_test.c
+++ head/lib/libc/tests/stdlib/strfmon_test.c
@@ -0,0 +1,71 @@
+/*-
+ * Copyright (C) 2018 Conrad Meyer <cem@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <locale.h>
+#include <monetary.h>
+#include <stdio.h>
+
+#include <atf-c.h>
+
+ATF_TC_WITHOUT_HEAD(strfmon_locale_thousands);
+ATF_TC_BODY(strfmon_locale_thousands, tc)
+{
+	char actual[40], expected[40];
+	struct lconv *lc;
+	const char *ts;
+	double n;
+
+	setlocale(LC_MONETARY, "sv_SE.UTF-8");
+
+	lc = localeconv();
+
+	ts = lc->mon_thousands_sep;
+	if (strlen(ts) == 0)
+		ts = lc->thousands_sep;
+
+	if (strlen(ts) < 2)
+		atf_tc_skip("multi-byte thousands-separator not found");
+
+	n = 1234.56;
+	strfmon(actual, sizeof(actual), "%i", n);
+
+	strcpy(expected, "1");
+	strlcat(expected, ts, sizeof(expected));
+	strlcat(expected, "234", sizeof(expected));
+
+	/* We're just testing the thousands separator, not all of strmon. */
+	actual[strlen(expected)] = '\0';
+	ATF_CHECK_STREQ(expected, actual);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, strfmon_locale_thousands);
+	return (atf_no_error());
+}