Add t('String to be translated') to your database I've altered a form where I set the save button text to t("Buy Product"); 

But how can I make sure that I can translate this string in admin/config/region/translate/translate ?

I've tried to add this

$conf['locale_custom_strings_en'][''] = array(
 'Buy Product' =&gt; 'Koop Product',
);


to my settings.php but no result. How can I make sure the string in my code can be translated in drupal itself?
Every single string passed to t($string) function will be added to the database, and you can find it in the path you montioned above "admin/config/regional/translate/translate"

clear the cache, and search it again with help of filters existing on the admin translate page.

[EDIT]

First of all a must have module in multilangual sites is the internationalization module, it's a powerfull and helpfull module...

But the most important thing in your case is that before you can see your string in the transltion page:


you must enable(switch to) the desired language
visit the page that contain that needle string
visit again the translation interface.


That's it you will find your string ;)