make ChapterDisplay use only one language at a time

Following the discussion in ietf-wg-cellar/ebml-specification#409
It could be assumed that English is always present because ChapLanguage is
mandatory and has a default value.

To avoid this interpretation we should have one language (and one IETF variant)
per ChapterDisplay item. We already have to use many ones per language. The merge
of different values because they have the same value is unlikely (but it may
exist in the real world).

ebml_matroska.xml

@@ -1165,14 +1165,14 @@ see (#physical-types) for a complete list of values.</documentation>
     <extension type="webmproject.org" webm="1"/>
     <extension type="libmatroska" cppname="ChapterString"/>
   </element>
-  <element name="ChapLanguage" path="\Segment\Chapters\EditionEntry\+ChapterAtom\ChapterDisplay\ChapLanguage" id="0x437C" type="string" default="eng" minOccurs="1">
+  <element name="ChapLanguage" path="\Segment\Chapters\EditionEntry\+ChapterAtom\ChapterDisplay\ChapLanguage" id="0x437C" type="string" default="eng" minOccurs="1" maxOccurs="1">
     <documentation lang="en" purpose="definition">The languages corresponding to the string,
 in the bibliographic ISO-639-2 form [@!ISO639-2].
 This Element **MUST** be ignored if the ChapLanguageIETF Element is used within the same ChapterDisplay Element.</documentation>
     <extension type="webmproject.org" webm="1"/>
     <extension type="libmatroska" cppname="ChapterLanguage"/>
   </element>
-  <element name="ChapLanguageIETF" path="\Segment\Chapters\EditionEntry\+ChapterAtom\ChapterDisplay\ChapLanguageIETF" id="0x437D" type="string" minver="4">
+  <element name="ChapLanguageIETF" path="\Segment\Chapters\EditionEntry\+ChapterAtom\ChapterDisplay\ChapLanguageIETF" id="0x437D" type="string" minver="4" maxOccurs="1">
     <documentation lang="en" purpose="definition">Specifies the language used in the ChapString according to [@!BCP47]
 and using the IANA Language Subtag Registry [@!IANALangRegistry].
 If this Element is used, then any ChapLanguage Elements used in the same ChapterDisplay **MUST** be ignored.</documentation>