Use SAX instead of the ElementTree

We just need to read and parse the file and create our own Python objects.
There's no need to construct the ElementTree (DOM tree) and go through it
afterwards to construct our objects.

This reduces the memory consumption roughly to a half at the cost of prolonged
"start" (import) time.

Also this way we don't need the python-lxml module.

Signed-off-by: Vratislav Podzimek <vpodzime@redhat.com>
Use 'trName' instead of 'name' for translated names

This simplifies SAX-based parsing and makes the XML file more lucid.

Signed-off-by: Vratislav Podzimek <vpodzime@redhat.com>
Clean self._iso639_1, self._iso639_2_t, and self._iso639_2_b at the end of a “language” element
Use “trName” instead of “name” also when writing the files