The Stripe popup that is displayed on your site is pulled directly from the Stripe API and isn't editable as the code is not accessable.
In the Stripe for RSMembership version 1.4.0 plugin we've added the language function from Stripes API to display the payment popup in the correct language.
The language setting is automatically set based on your sites language settings in the Joomla Language Manager. Note that not all languages are available from Stripe and if you have Joomla set to display a language that is not available it will default to English.
Stripe currenty supports the following languages: