Use BCP47 language codes in citeproc tests.

test/command/pandoc-citeproc-65.md

@@ -14,7 +14,7 @@ references:
   id: 'stotz:1996handbuch'
   issued:
     literal: 1996_2004
-  language: German
+  language: de
   number-of-volumes: 5
   publisher: Beck
   publisher-place: Munich

test/command/pandoc-citeproc-70.md

@@ -22,7 +22,7 @@ references:
   issued:
     date-parts:
     - - 2009
-  language: French
+  language: fr
   page: '155--174'
   publisher: 'Fédération Internationale des Instituts d''Études
     Médiévales'
@@ -42,7 +42,7 @@ references:
   issued:
     date-parts:
     - - 1955
-  language: Latin
+  language: la
   note: 'ArticleType: research-article / Full publication date: 1955 /
     Copyright © 1955 Fordham University'
   page: '163--193'

test/command/unicode-collation.md

@@ -108,7 +108,7 @@ Ubina, A. J. (1985).
 % pandoc -C -t plain
 ---
 nocite: '@*'
-lang: fr
+lang: fr-FR-u-kb-true
 references:
 - id: cote
   author: cote