I think I know what you're getting at. You look to have 2 misconceptions about the rewrite engine. Here's two facts to set you straight: patterns don't match against query strings (the documentation isn't very clear on this) and if you specify a query string in a substitution, it will replace the current query string (the documentation mentions this in the description of the QSA flag). Use the QSA flag on a rule will make the rewrite engine append to the query string rather than replacing it.
Also, your first RewriteRule would apply anytime the second would. As the first alters the URL, the second RewriteRule will never match.
And another thing: you don't have grouping operators (i.e. parenthesis) in your RewriteCond, so the "%1" in the following RewriteRule will be empty.
I thing this is what you're looking for:
Code:
RewriteEngine On
RewriteRule ^/?(en\w+)/(.+)$ /claimer/$2?world=$1 [QSA]
That should turn /en10/calculator.php?page=bbcode into /claimer/calculator.php?page=bbcode&world=en10