Regional language differences just mean you use a languagecode that includes regions. There are many many variations of English but each has different region codes (as do the dictionaries).
en_US vs en_GB vs en_CA are different dictionaries targeted for US, Great Britain, and Canada respectively. The dictionaries names all start that way.
The dc:language code allows the region to be used: en-US instead of just en
The xml:lang and lang attributes also allow a region to be specified: "en-GB" instead of just "en".
The language pulldown supports a large set of regional language codes. Here is a code snippet for just Spanish:
Code:
"es" << tr("Spanish") <<
"es-AR" << tr("Spanish") + QString(" - ") + tr("Argentina") <<
"es-BO" << tr("Spanish") + QString(" - ") + tr("Bolivia") <<
"es-CL" << tr("Spanish") + QString(" - ") + tr("Chile") <<
"es-CO" << tr("Spanish") + QString(" - ") + tr("Columbia") <<
"es-CR" << tr("Spanish") + QString(" - ") + tr("Costa Rica") <<
"es-DO" << tr("Spanish") + QString(" - ") + tr("Dominican Republic") <<
"es-EC" << tr("Spanish") + QString(" - ") + tr("Ecuador") <<
"es-SV" << tr("Spanish") + QString(" - ") + tr("El Salvador") <<
"es-GT" << tr("Spanish") + QString(" - ") + tr("Guatemala") <<
"es-HN" << tr("Spanish") + QString(" - ") + tr("Honduras") <<
"es-MX" << tr("Spanish") + QString(" - ") + tr("Mexico") <<
"es-NI" << tr("Spanish") + QString(" - ") + tr("Nicaragua") <<
"es-PA" << tr("Spanish") + QString(" - ") + tr("Panama") <<
"es-PY" << tr("Spanish") + QString(" - ") + tr("Paraguay") <<
"es-PE" << tr("Spanish") + QString(" - ") + tr("Peru") <<
"es-PR" << tr("Spanish") + QString(" - ") + tr("Puerto Rico") <<
"es-ES" << tr("Spanish") + QString(" - ") + tr("Spain") <<
"es-UY" << tr("Spanish") + QString(" - ") + tr("Uruguay") <<
"es-VE" << tr("Spanish") + QString(" - ") + tr("Venezuela") <<
As for adding this to the user manual, the number of users who do not use the normal hunspell dictionaries is very very small. If people do run into difficulties they can of course come here to our User Forum on Mobileread to get help.
That said, if you would like to help edit the user-guide with additional information, we would be happy to consider it for inclusion.
Hope this helps!