diff --git a/src/calibre/gui2/tweak_book/__init__.py b/src/calibre/gui2/tweak_book/__init__.py
index 621c45eff2..220aeb2167 100644
--- a/src/calibre/gui2/tweak_book/__init__.py
+++ b/src/calibre/gui2/tweak_book/__init__.py
@@ -10,6 +10,8 @@ import string
 from future_builtins import map
 
 from calibre.utils.config import JSONConfig
+from calibre.spell.dictionary import Dictionaries, parse_lang_code
+
 tprefs = JSONConfig('tweak_book_gui')
 d = tprefs.defaults
 
@@ -69,3 +71,12 @@ class NonReplaceDict(dict):
 actions = NonReplaceDict()
 editors = NonReplaceDict()
 TOP = object()
+dictionaries = Dictionaries()
+
+def set_book_locale(lang):
+    try:
+        dictionaries.default_locale = parse_lang_code(lang)
+        if dictionaries.default_locale.langcode == 'und':
+            raise ValueError('')
+    except ValueError:
+        dictionaries.default_locale = dictionaries.ui_locale
diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index ba168326b6..68ea275085 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -27,7 +27,8 @@ from calibre.ebooks.oeb.polish.toc import remove_names_from_toc, find_existing_t
 from calibre.ebooks.oeb.polish.utils import link_stylesheets, setup_cssutils_serialization as scs
 from calibre.gui2 import error_dialog, choose_files, question_dialog, info_dialog, choose_save_file
 from calibre.gui2.dialogs.confirm_delete import confirm
-from calibre.gui2.tweak_book import set_current_container, current_container, tprefs, actions, editors
+from calibre.gui2.tweak_book import (
+    set_current_container, current_container, tprefs, actions, editors, set_book_locale)
 from calibre.gui2.tweak_book.undo import GlobalUndoHistory
 from calibre.gui2.tweak_book.file_list import NewFileDialog
 from calibre.gui2.tweak_book.save import SaveManager, save_container, find_first_existing_ancestor
@@ -237,6 +238,7 @@ class Boss(QObject):
         set_current_container(container)
         with BusyCursor():
             self.current_metadata = self.gui.current_metadata = container.mi
+            set_book_locale(self.current_metadata.language)
             self.global_undo.open_book(container)
             self.gui.update_window_title()
             self.gui.file_list.current_edited_name = None
@@ -732,6 +734,7 @@ class Boss(QObject):
             f.write(ed.data)
         if name == container.opf_name:
             container.refresh_mime_map()
+            set_book_locale(container.mi.language)
         if container is current_container():
             ed.is_synced_to_container = True
             if name == container.opf_name:
diff --git a/src/calibre/spell/dictionary.py b/src/calibre/spell/dictionary.py
index fd5939c58e..acc5516952 100644
--- a/src/calibre/spell/dictionary.py
+++ b/src/calibre/spell/dictionary.py
@@ -34,6 +34,7 @@ def get_codes():
     return ccodes, ccodemap
 
 def parse_lang_code(raw):
+    raw = raw or ''
     parts = raw.replace('_', '-').split('-')
     lc = canonicalize_lang(parts[0])
     if lc is None:
@@ -161,6 +162,7 @@ class Dictionaries(object):
             self.default_locale = parse_lang_code(get_lang())
         except ValueError:
             self.default_locale = parse_lang_code('en-US')
+        self.ui_locale = self.default_locale
 
     def clear_caches(self):
         self.dictionaries.clear(), self.word_cache.clear()