PICARD-1589: Support language for ID3 comments.

Adds syntax "comment:{language}:{description}" in addition to existing "comment:{description}" for comment tag names.

test/formats/common.py

@@ -87,6 +87,7 @@ TAGS = {
     'catalognumber': 'Foo',
     'comment:': 'Foo',
     'comment:foo': 'Foo',
+    'comment:deu:foo': 'Foo',
     'compilation': '1',
     'composer': 'Foo',
     'composersort': 'Foo',

test/formats/test_id3.py

@@ -101,14 +101,18 @@ class CommonId3Tests:
         def test_comment_delete(self):
             metadata = Metadata(self.tags)
             metadata['comment:bar'] = 'Foo'
+            metadata['comment:XXX:withlang'] = 'Foo'
             original_metadata = save_and_load_metadata(self.filename, metadata)
             del metadata['comment:bar']
+            del metadata['comment:XXX:withlang']
             new_metadata = save_and_load_metadata(self.filename, metadata)
 
             self.assertIn('comment:foo', original_metadata)
             self.assertIn('comment:bar', original_metadata)
+            self.assertIn('comment:XXX:withlang', original_metadata)
             self.assertIn('comment:foo', new_metadata)
             self.assertNotIn('comment:bar', new_metadata)
+            self.assertNotIn('comment:XXX:withlang', new_metadata)
 
         @skipUnlessTestfile
         def test_id3v23_simple_tags(self):

test/test_util_tags.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+from test.picardtestcase import PicardTestCase
+
+from picard.util.tags import (
+    display_tag_name,
+    parse_comment_tag,
+)
+
+
+class UtilTagsTest(PicardTestCase):
+    def test_display_tag_name(self):
+        self.assertEqual('Artist', display_tag_name('artist'))
+        self.assertEqual('Lyrics', display_tag_name('lyrics:'))
+        self.assertEqual('Comment [Foo]', display_tag_name('comment:Foo'))
+
+    def test_parse_comment_tag(self):
+        self.assertEqual(('XXX', 'foo'), parse_comment_tag('comment:XXX:foo'))
+        self.assertEqual(('eng', 'foo'), parse_comment_tag('comment:foo'))