diff -r d07300e8a14d -r ee4cf96a9406 src/Pure/Tools/spell_checker.scala
--- a/src/Pure/Tools/spell_checker.scala	Thu Jun 01 21:24:33 2017 +0200
+++ b/src/Pure/Tools/spell_checker.scala	Thu Jun 01 21:43:36 2017 +0200
@@ -57,7 +57,7 @@
 
   class Dictionary private[Spell_Checker](val path: Path)
   {
-    val lang = path.split_ext._1.base.implode
+    val lang = path.split_ext._1.base_name
     val user_path = Path.explode("$ISABELLE_HOME_USER/dictionaries") + Path.basic(lang)
     override def toString: String = lang
   }