Magento – Different domain for different store view (language)

Some information for you on this:

The language is set in the getPdf() method of Mage_Sales_Model_Order_Pdf_Invoice. You will see that the locale and store are set according to the invoide data:

Mage::app()->getLocale()->emulate($invoice->getStoreId());
Mage::app()->setCurrentStore($invoice->getStoreId());

What you could do is to extend this functionality and add maybe a second button to your admin-backend which always generate the english invoice.

Be aware that emulate() needs a store id. If you don't have an english store, this won`t work.

In this case you could try to remove the first line Mage::app()->getLocale()->emulate($invoice->getStoreId()); from your rewritten-method and also the revert back Mage::app()->getLocale()->revert(); at the end of the function which sets the locale back to the last one before emulation.

The catalog price rules are on website level, mainly because the price can be on a global or website level. You cannot have different prices and different catalog price rules on the store view level.
An option would be to transform your 3 store view into 3 websites with one store view each.
This should be simple enough.
Just create 2 more websites and associate 2 of the store views to them...one for each.
Then you have to bulk update the products to associate them to all the websites.
You can do that from the product grid, buy selecting all of them and selecting "Change attributes" from the top right select.

Then you will be able to add your discount rules as you wish.