GUACAMOLE-422: Define field type which represents the ID of any supported language.

guacamole-ext/src/main/java/org/apache/guacamole/form/Field.java

@@ -91,6 +91,14 @@ public class Field {
          */
         public static String TIMEZONE = "TIMEZONE";
 
+        /**
+         * Field type which allows selection of languages. The languages
+         * displayed are the set of languages supported by the Guacamole web
+         * application. Legal values are valid language IDs, as dictated by
+         * the filenames of Guacamole's available translations.
+         */
+        public static String LANGUAGE = "LANGUAGE";
+
         /**
          * A date field whose legal values conform to the pattern "YYYY-MM-DD",
          * zero-padded.

guacamole-ext/src/main/java/org/apache/guacamole/form/LanguageField.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.guacamole.form;
+
+/**
+ * Represents a language field. The field may contain only valid language
+ * identifiers as used by the Guacamole web application for its translations.
+ * Language identifiers are defined by the filenames of the JSON files
+ * containing the translation.
+ */
+public class LanguageField extends Field {
+
+    /**
+     * Creates a new LanguageField with the given name.
+     *
+     * @param name
+     *     The unique name to associate with this field.
+     */
+    public LanguageField(String name) {
+        super(name, Field.Type.LANGUAGE);
+    }
+
+    /**
+     * Parses the given string into a language ID string. As any string may be
+     * a valid language ID as long as it has a corresponding translation, the
+     * only transformation currently performed by this function is to ensure
+     * that a blank language string is parsed into null.
+     *
+     * @param language
+     *     The language string to parse, which may be null.
+     *
+     * @return
+     *     The ID of the language corresponding to the given string, or null if
+     *     if the given language string was null or blank.
+     */
+    public static String parse(String language) {
+
+        // Return null if no language is provided
+        if (language == null || language.isEmpty())
+            return null;
+
+        // Otherwise, assume language is already a valid language ID
+        return language;
+
+    }
+
+}