Htaccess new rewrite rule

[h3llc0re]

Hello,
i need help with adding a new rewrite rule in .htaccess in prestashop. I've generated my htaccess file with prestashop and found two lines which i think a need edit:

RewriteRule ^cs$ /prestashop/cs/ [QSA,L]
RewriteRule ^cs/([?&]*)$ /prestashop/$1?isolang=cs [QSA,L]

Because i don't know htaccess syntax and i need it i'm asking you. I need to put a new _GET variable right after variable isolang but don't know how, can you help me?

Thanks,
Jakub.