A One Page (sometimes called One Step) Checkout implementation on eCommerce websites reduces the number of steps in conversion. It shortens the funnel, increases conversion and reduces cart abandonment. When we replace standard checkout with OPC, we loose functionality to save abandoned carts as prospects. These prospects can be 'incentivized' later to complete transaction. It is recommended to implement save prospect on minimum data entered in tandem with One Page Checkout. We have written a blog article on it which explains OPC in even more detail.
We are eCommerce/Magento implementation experts and have consultants who can help you implement features like this that will impact your topline.
There are varying degrees to which you can enable/implement localization/internationalization. In simplest form, you can use google translate to translate page in multiple languages. This is far from perfect approach. Alternately, a multi-language enabled website will allow you to enter translations. There is more to it than that. What about text in graphics/images? Same thing applies to currency. In simplest form one can convert currency from USD to local current using a conversion rate. But for a global company, the price at which you may want to sell in US market vs Europe may be different. The taxes/VAT may be different. UK/Europe stores display prices including VAT. To summarize, for a good implementation, consult a localization and internationalization expert.