Rewrite you can do this magento two process

Process One:

one is rewrite the class Mage_Checkout_CartController and in this addAction() at code if (!$cart->getQuote()->getHasError()) { add redirection code $this->_redirect('checkout/onepage');

Process two:

Using event/Observer

Event:

checkout_cart_add_product_complete

This event provide the data of product object and cart request parameters

Mage::dispatchEvent('checkout_cart_add_product_complete',
    array('product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse())
);

Config.xml code:

<frontend>
    <events>
    <checkout_cart_add_product_complete>
             <observers>
             <packagename_modulename_observer>
                  <type>singleton</type>
                  <class>packagename_modulename/observer</class>
                  <method>addToCartComplete</method>
             </packagename_modulename_observer>
             </observers>
         </checkout_cart_add_product_complete>
     </events>
</frontend>

Observer code:

using this event you can set your return url

$observer->getEvent()->getRequest()->setParam('return_url',Mage::getUrl('checkout/onepage')); this redirect to you at checkout Onepage function:

public function addToCartComplete(Varien_Event_Observer $observer) {
 $product = $observer->getEvent()->getProduct();
 $Request=$observer->getEvent()->getRequest();
 $Request->setParam('return_url',Mage::getUrl('checkout/onepage'));
}

this redirect to you at Checkout onepage

So to translate the text you can do a few things.

1. Inline translations: you can set-up inline translations in the admin section then when you visit the frontend you can then translate strings and translate the button text.
2. Add a locale file to your theme: if in your theme you add a locale file you can then add any translations that you wish.
3. Update the template in your theme: if you add the template to your theme you can simply change the text shown.

I would go with the theme locale option here. Just create a file public/app/design/frontend/your_package/your_theme/locale/en_US/translate.csv then in this file translate your string.