Add Vietnamese translation

[tnhung2011]

Vietnamese translation for the project. Some notes though:

• The translation may not be completely similar to the base English version though, as I have rewritten on some parts of the translation basing around the original English one to fit with caveats of Vietnamese
• In use.html, I have borrowed some paragraphs about using the pool on Windows from the (untranslated) English text in the Danish files. IMO the command snippets (combined with UI solution) should be included as alternatives to other methods (in case it doesn't work)

(NB! You could use Google Translate to content-check my translation - it understands concepts of Vietnamese pretty well)