Correct few defaults of spell-checking's layer

- Disable `auto-dictionary-mode` by default. Add a layer's variable to
  enable it. Its behaviour has some impact on user-defined dictionary
  preferences, it has some bugs in daemon mode, and not all languages
  are supported. It's why it better to have it disabled by default.

- Toggle off `auto-dictionary-mode` when spell-checking is toggled off.

- Call `flyspell-buffer` when toggling **on** spell-checking to reveal
  mistakes (probably the wanted behaviour when activating
  spell-checking)

- When `auto-dictionary-mode` is enabled, if a buffer's dictionary is
  manually changed with `SPC S d`, it is restored after toggling
  spell-checking on/off, otherwise `auto-dictionary` will thy to guess
  it again.

layers/spell-checking/README.org

@@ -6,6 +6,7 @@
    - [[Layer][Layer]]
    - [[Spell Checker Configuration][Spell Checker Configuration]]
    - [[Disabling by default][Disabling by default]]
+   - [[Enabling auto-dictionary-mode][Enabling auto-dictionary-mode]]
  - [[Key Bindings][Key Bindings]]
 
 * Description
@@ -38,6 +39,17 @@ toggled off with ~SPC t S~. You can default this to off by setting the variable
   '((spell-checking :variables spell-checking-enable-by-default nil)))
 #+END_SRC
 
+** Enabling auto-dictionary-mode
+=auto-dictionary-mode= tries to detect the current language from the buffer
+content, and activate the corresponding dictionary. You can enable it by setting
+the variable =spell-checking-enable-auto-dictionary= to something other than
+=nil=:
+
+#+BEGIN_SRC emacs-lisp
+(setq-default dotspacemacs-configuration-layers
+  '((spell-checking :variables spell-checking-enable-auto-dictionary t)))
+#+END_SRC
+
 * Key Bindings
 
 | Key Binding | Description              |

layers/spell-checking/config.el

@@ -18,3 +18,6 @@
 
 (defvar spell-checking-enable-by-default t
   "Enable spell checking by default.")
+
+(defvar spell-checking-enable-auto-dictionary nil
+  "Specify if auto-dictionary should be enabled or not.")

layers/spell-checking/packages.el

@@ -20,6 +20,7 @@
 (defun spell-checking/init-auto-dictionary ()
   (use-package auto-dictionary
     :defer t
+    :if spell-checking-enable-auto-dictionary
     :init
     (add-hook 'flyspell-mode-hook 'auto-dictionary-mode)))
 
@@ -36,8 +37,21 @@
 
       (spacemacs|add-toggle spelling-checking
         :status flyspell-mode
-        :on (flyspell-mode)
-        :off (flyspell-mode -1)
+        :on
+        (progn
+          (flyspell-mode)
+          ;; Redefine the buffer local dictionary if it was set, otherwise
+          ;; auto-dictionary will replace it with guessed one.
+          (when (and (fboundp 'adict-change-dictionary) ispell-local-dictionary)
+            (adict-change-dictionary ispell-local-dictionary))
+          ;; Reanalyze the whole buffer to show mistakes. It's probably the
+          ;; wanted behaviour when activating spell-checking.
+          (flyspell-buffer))
+        :off
+        (progn
+          (flyspell-mode -1)
+          ;; Also disable auto-dictionary when disabling spell-checking.
+          (when (fboundp 'auto-dictionary-mode) (auto-dictionary-mode -1)))
         :documentation
         "Enable automatic spell checking."
         :evil-leader "tS")