let me first clarify a couple of things;
1./ The admin interface/control panel and system messages language is selected at /go by clicking the little flag before logging in. This is session based. It means one user can log in English while another one logs in using a German interface.
2./ Website's operational language is set from the control panel, at OTHERS/config. This is the language that this website is built to served.
This is used to display messages whenever the system needs to interact with your website visitors, default buttons labels and the such.
Both systems use the same source file found on your server/install at seotoaster/system/languages
You can copy a lng in that folder and then rename it using the Zend accepted country code at http://framework.zend.com/manual/1.12/en/zend.locale.appendix.html
. In your case it would be nl.lng.
Then once your translation is ready, add a little flat into the folder /system/images/flags/
in your case it would be nl.png to make it appear under the /go interface, and allow administrators to log in.
We appreciate any correction you can bring to existing lng files, and will certainly credit your efforts.
As to create a website to cater to multiple markets. Our approach is to favor multiple system installs (they can all use 1 core to facilitate updates and show various uversions of the site in subfolders) to allow for url and all variable optimization to cater to their respective markets.
Let us know if anything is unclear.
Thanks for using seotoaster.