Florian noticed that this results in art which does not spark joy.

	$ ftp https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-demo.txt
	$ watch cat UTF-8-demo.txt

I took inspiration from tmux/tmux.c.

OK?

Index: watch.c
===================================================================
RCS file: /cvs/src/usr.bin/watch/watch.c,v
diff -u -p -r1.23 watch.c
--- watch.c	21 May 2025 08:32:10 -0000	1.23
+++ watch.c	21 May 2025 12:15:54 -0000
@@ -26,6 +26,7 @@
 #include <err.h>
 #include <errno.h>
 #include <event.h>
+#include <langinfo.h>
 #include <locale.h>
 #include <paths.h>
 #include <signal.h>
@@ -124,7 +125,17 @@ main(int argc, char *argv[])
 	struct event ev_sigint, ev_sighup, ev_sigterm, ev_sigwinch, ev_stdin;
 	size_t len, rem;
 	int i, ch;
+	const char *s;
 	char *p;
+
+	if (setlocale(LC_CTYPE, "en_US.UTF-8") == NULL &&
+	    setlocale(LC_CTYPE, "C.UTF-8") == NULL) {
+		if (setlocale(LC_CTYPE, "") == NULL)
+			errx(1, "invalid LC_ALL, LC_CTYPE or LANG");
+		s = nl_langinfo(CODESET);
+		if (strcasecmp(s, "UTF-8") != 0 && strcasecmp(s, "UTF8") != 0)
+			errx(1, "need UTF-8 locale (LC_CTYPE) but have %s", s);
+        }
 
 	while ((ch = getopt(argc, argv, "cls:wx")) != -1)
 		switch (ch) {