first, strip '.UTF-8' is needed for almost all apps using gettext, i found this: http://www.openwall.com/lists/musl/2016/05/11/81, but it seems nothing was added in musl.
--- musl-1.1.16/src/locale/dcngettext.c	2017-01-01 03:27:17.000000000 +0000
+++ musl-1.1.16/src/locale/dcngettext.c	2017-01-01 03:27:17.000000000 +0000
@@ -121,6 +121,7 @@
 	struct __locale_struct *loc = CURRENT_LOCALE;
 	const struct __locale_map *lm;
 	const char *dirname, *locname, *catname;
+	char *posu;
 	size_t dirlen, loclen, catlen, domlen;

 	if ((unsigned)category >= LC_ALL) goto notrans;
@@ -144,7 +143,10 @@

 	catname = catnames[category];
 	catlen = catlens[category];
-	loclen = strlen(locname);
+	if( posu = strstr(locname, ".UTF-8") )
+		loclen = posu - locname;
+	else
+		loclen = strlen(locname);

 	size_t namelen = dirlen+1 + loclen+1 + catlen+1 + domlen+3;
 	char name[namelen+1], *s = name;

second, i compiled fcitx with musl's intl, parts of text is translated but other parts. I gone through the whole progranm, finally found this mistake. p refers to the list to add, so the return value of strcmp() is always 0, when bindtextdomain executes more than onetime, active flag is set to 0 wrongly.
--- musl-1.1.16/src/locale/dcngettext.c	2017-01-20 10:43:48.054441176 +0000
+++ musl-1.1.16/src/locale/dcngettext.c	2017-01-20 10:43:48.054441176 +0000
@@ -74,7 +74,7 @@
 	a_store(&p->active, 1);

 	for (q=bindings; q; q=q->next) {
-		if (!strcmp(p->domainname, domainname) && q != p)
+		if (!strcmp(q->domainname, domainname) && q != p)
 			a_store(&q->active, 0);
 	}