Change "Language search" to "Search for a language"

Suggested at https://phabricator.wikimedia.org/T138235 .
This brings ULS closer to the mobile language search interface
in MediaWiki.

i18n/en.json

@@ -18,5 +18,5 @@
 	"uls-common-languages": "Suggested languages",
 	"uls-no-results-suggestion-title": "You may be interested in:",
 	"uls-search-help": "You can search by language name, script name, ISO code of language or you can browse by region.",
-	"uls-search-placeholder": "Language search"
+	"uls-search-placeholder": "Search for a language"
 }