Install Aspell and all the Aspell dictionaries you need.
The Windows port is not currently maintained, but you can get
the 0.5 version from http://aspell.net/win32/
This has installers for the program and the dictionaries.

Activate spellchecking by setting two properties in doc/doc.build.user.

<property name="spellcheck" value="true"/> 
<property name="path.spellcheck" value="C:\Path\to\your\Aspell.exe"/>

You may have to use the DOS 8.3 version if the path includes spaces,
as the Aspell.bat batch file doesn't handle them well.
eg. for the default installation use:
<property name="path.spellcheck" value="C:\Progra~1\Aspell\bin\aspell.exe"/>

This folder contains spellchecker dictionaries for Aspell. 
One for each language and a generic template file that is copied to each language.

TortoiseGit.tmpl.pws:
  Generic dictionary of terms & Keywords used in the TortoiseGit manual. 
  This file is language independent and is copied to each target language 
  (named temp.pws), when the translation takes place.

de.pws, en.pws, ...:
  Dictionaries of terms for each language.

The process is still kludgy:
- Aspell doesn't work for Asian languages (yet). No fix available.
- It needs a batch file to run Aspell from nant due to IO 
  redirection problems. No fix found for this.
- It will fail if <NN>.pws doesn't exist yet. 
  Create a text file <NN>.pws that just contains the line:
  "personal_ws-1.1 <NN> 0" where <NN> is the country code.
  Look in de.pws or en.pws for examples. 
  Add your terms after this line.

The output of the spellchecking process is written to "spellcheck_<NN>.log".
