If -u is specified or UTF-8 is otherwise detected when the server is started,

enable the utf8 and status-utf8 optons. While here, note in the man page that
the server is started with the first session and exits when none remain.

tmux.1

@@ -36,11 +36,13 @@ controlled from a single terminal.
 .Pp
 .Nm
 runs as a server-client system.
-A server is created automatically when necessary and holds a number of
+A server holds a number of
 .Em sessions ,
 each of which may have a number of
 .Em windows
 linked to it.
+A server is started automatically when the first session is created and exits
+when all the sessions it contains are destroyed.
 A window may be split on screen into one or more
 .Em panes ,
 each of which is a separate terminal.
@@ -123,6 +125,14 @@ This is not always correct: the
 flag explicitly informs
 .Nm
 that UTF-8 is supported.
+.Pp
+If the server is started from a client passed
+.Fl u
+or where UTF-8 is detected, the
+.Ic utf8
+and
+.Ic status-utf8
+options are enabled in the global window and session options respectively.
 .It Fl v
 Request verbose logging.
 This option may be specified multiple times for increasing verbosity.

tmux.c

@@ -266,6 +266,22 @@ main(int argc, char **argv)
 	log_open_tty(debug_level);
 	siginit();
 
+	if (!(flags & IDENTIFY_UTF8)) {
+		/*
+		 * If the user has set whichever of LC_ALL, LC_CTYPE or LANG
+		 * exist (in that order) to contain UTF-8, it is a safe
+		 * assumption that either they are using a UTF-8 terminal, or
+		 * if not they know that output from UTF-8-capable programs may
+		 * be wrong.
+		 */
+		if ((s = getenv("LC_ALL")) == NULL) {
+			if ((s = getenv("LC_CTYPE")) == NULL)
+				s = getenv("LANG");
+		}
+		if (s != NULL && strcasestr(s, "UTF-8") != NULL)
+			flags |= IDENTIFY_UTF8;
+	}
+
 	options_init(&global_s_options, NULL);
 	options_set_number(&global_s_options, "bell-action", BELL_ANY);
 	options_set_number(&global_s_options, "buffer-limit", 9);
@@ -292,7 +308,10 @@ main(int argc, char **argv)
 	options_set_string(&global_s_options, "status-left", "[#S]");
 	options_set_string(
 	    &global_s_options, "status-right", "\"#24T\" %%H:%%M %%d-%%b-%%y");
-	options_set_number(&global_s_options, "status-utf8", 0);
+	if (flags & IDENTIFY_UTF8)
+		options_set_number(&global_s_options, "status-utf8", 1);
+	else
+		options_set_number(&global_s_options, "status-utf8", 0);
 
 	options_init(&global_w_options, NULL);
 	options_set_number(&global_w_options, "aggressive-resize", 0);
@@ -309,29 +328,16 @@ main(int argc, char **argv)
 	options_set_number(&global_w_options, "mode-keys", MODEKEY_EMACS);
 	options_set_number(&global_w_options, "monitor-activity", 0);
 	options_set_string(&global_w_options, "monitor-content", "%s", "");
-	options_set_number(&global_w_options, "utf8", 0);
+	if (flags & IDENTIFY_UTF8)
+		options_set_number(&global_w_options, "utf8", 1);
+	else
+		options_set_number(&global_w_options, "utf8", 0);
 	options_set_number(&global_w_options, "window-status-attr", 0);
 	options_set_number(&global_w_options, "window-status-bg", 8);
 	options_set_number(&global_w_options, "window-status-fg", 8);
 	options_set_number(&global_w_options, "xterm-keys", 0);
  	options_set_number(&global_w_options, "remain-on-exit", 0);
 
-	if (!(flags & IDENTIFY_UTF8)) {
-		/*
-		 * If the user has set whichever of LC_ALL, LC_CTYPE or LANG
-		 * exist (in that order) to contain UTF-8, it is a safe
-		 * assumption that either they are using a UTF-8 terminal, or
-		 * if not they know that output from UTF-8-capable programs may
-		 * be wrong.
-		 */
-		if ((s = getenv("LC_ALL")) == NULL) {
-			if ((s = getenv("LC_CTYPE")) == NULL)
-				s = getenv("LANG");
-		}
-		if (s != NULL && strcasestr(s, "UTF-8") != NULL)
-			flags |= IDENTIFY_UTF8;
-	}
-
 	if (cfg_file == NULL) {
 		home = getenv("HOME");
 		if (home == NULL || *home == '\0') {