Fix/bug 1691 preserving change language binding (#1720)

* test: bug 1691 second attempt, add test for actual use of i18n.changeLanguage with useTranslation

* fix: (pre-cleanup) bug 1691 cherry pick of 168b8f0: use memoization of fixedT function,

bad impl, breaks test that was introduced in parent commit, verifying bug here: #1716 (comment)

* fix: (pre-cleanup) bug 1691 fixing dev issue: regenerate a new t fn everywhere

other than where the bug was occurring, and keep shallow routing working via dependency list of useCallback hook.  Fixes test broken by parent commit.

* style: cleanup debug messages for bug 1691

* build: bug 1691 npm run build

react-i18next.js

@@ -612,6 +612,12 @@
 	  }, [value, ignore]);
 	  return ref.current;
 	};
+	function alwaysNewT(i18n, language, namespace, keyPrefix) {
+	  return i18n.getFixedT(language, namespace, keyPrefix);
+	}
+	function useMemoizedT(i18n, language, namespace, keyPrefix) {
+	  return react.useCallback(alwaysNewT(i18n, language, namespace, keyPrefix), [i18n, language, namespace, keyPrefix]);
+	}
 	function useTranslation(ns) {
 	  let props = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : {};
 	  const {
@@ -650,9 +656,9 @@
 	  namespaces = typeof namespaces === 'string' ? [namespaces] : namespaces || ['translation'];
 	  if (i18n.reportNamespaces.addUsedNamespaces) i18n.reportNamespaces.addUsedNamespaces(namespaces);
 	  const ready = (i18n.isInitialized || i18n.initializedStoreOnce) && namespaces.every(n => hasLoadedNamespace(n, i18n, i18nOptions));
-	  function getT() {
-	    return i18n.getFixedT(props.lng || null, i18nOptions.nsMode === 'fallback' ? namespaces : namespaces[0], keyPrefix);
-	  }
+	  const memoGetT = useMemoizedT(i18n, props.lng || null, i18nOptions.nsMode === 'fallback' ? namespaces : namespaces[0], keyPrefix);
+	  const getT = () => memoGetT;
+	  const getNewT = () => alwaysNewT(i18n, props.lng || null, i18nOptions.nsMode === 'fallback' ? namespaces : namespaces[0], keyPrefix);
 	  const [t, setT] = react.useState(getT);
 	  let joinedNS = namespaces.join();
 	  if (props.lng) joinedNS = `${props.lng}${joinedNS}`;
@@ -667,19 +673,19 @@
 	    if (!ready && !useSuspense) {
 	      if (props.lng) {
 	        loadLanguages(i18n, props.lng, namespaces, () => {
-	          if (isMounted.current) setT(getT);
+	          if (isMounted.current) setT(getNewT);
 	        });
 	      } else {
 	        loadNamespaces(i18n, namespaces, () => {
-	          if (isMounted.current) setT(getT);
+	          if (isMounted.current) setT(getNewT);
 	        });
 	      }
 	    }
 	    if (ready && previousJoinedNS && previousJoinedNS !== joinedNS && isMounted.current) {
-	      setT(getT);
+	      setT(getNewT);
 	    }
 	    function boundReset() {
-	      if (isMounted.current) setT(getT);
+	      if (isMounted.current) setT(getNewT);
 	    }
 	    if (bindI18n && i18n) i18n.on(bindI18n, boundReset);
 	    if (bindI18nStore && i18n) i18n.store.on(bindI18nStore, boundReset);