Add support of form attribute for detached input

Input elements are usually created below a form to be submitted.
There's another alternative with the `form` attribute that allows
them to be placed anywhere in the document.

This field was ignored by qTranslate, assuming all input below form.
This problem was first seen with ACF6 field group title (#1252).
This patches considers this attribute before creating new elements.
It is read in priority, before looking for the closest form.

admin/js/core/qtranslatex.js

@@ -207,8 +207,23 @@ const qTranslateX = function (pg) {
             }
         }
 
-        let contents;
+        const inputFieldFormId = $(inputField).attr('form');
+        const $form = (inputFieldFormId !== undefined) ? $('#' + inputFieldFormId) : $(inputField).closest('form');
+        if (!$form.length) {
+            console.error('No form found for translatable field id=', inputField.id);
+            return;
+        }
+        const form = $form[0];
+        const formContainsInput = ($(inputField).closest($form).length > 0);
+        const addElementToForm = function (newField) {
+            if (formContainsInput) {
+                inputField.parentNode.insertBefore(newField, inputField);
+            } else {
+                $form.append(newField);
+            }
+        }
 
+        let contents;
         hook.fields = {};
         if (!qTranslateConfig.RAW) {
             // Most crucial moment when untranslated content is parsed
@@ -224,22 +239,17 @@ const qTranslateX = function (pg) {
                     newName += suffixName;
                 const newField = qtranxj_ce('input', {name: newName, type: 'hidden', className: 'hidden', value: text});
                 hook.fields[lang] = newField;
-                inputField.parentNode.insertBefore(newField, inputField);
+                addElementToForm(newField);
             }
 
             // insert a hidden element in the form so that the edit language is sent to the server
-            const $form = $(inputField).closest('form');
-            if ($form.length) {
-                const $hidden = $form.find('input[name="qtranslate-edit-language"]');
-                if (!$hidden.length) {
-                    qtranxj_ce('input', {
-                        type: 'hidden',
-                        name: 'qtranslate-edit-language',
-                        value: qTranslateConfig.activeLanguage
-                    }, $form[0], true);
-                }
-            } else {
-                console.error('No form found for translatable field id=', inputField.id);
+            const $hidden = $form.find('input[name="qtranslate-edit-language"]');
+            if (!$hidden.length) {
+                qtranxj_ce('input', {
+                    type: 'hidden',
+                    name: 'qtranslate-edit-language',
+                    value: qTranslateConfig.activeLanguage
+                }, form, true);
             }
         }
 
@@ -271,8 +281,9 @@ const qTranslateX = function (pg) {
                 break;
         }
 
-        if (hook.sepfield)
-            inputField.parentNode.insertBefore(hook.sepfield, inputField);
+        if (hook.sepfield) {
+            addElementToForm(hook.sepfield);
+        }
 
         return hook;
     };