Fix language codes like zh_TW for tutorials
As #59 (closed) described, in tutorials we got wrong language codes if they consist of two elements:
<html lang="pt_BR">
instead of:
<html lang="pt-BR">
Affected: pt_BR, zh_CN, zh_TW
The problem was that we get language codes from filenames. Now we get from XML since itstool
provides valid languages codes.
Partially fix for #59 (closed)