Greasy Fork is available in English.
Pops up the Steam product activation dialog when copying keys from bundle/reseller sites. Supports various bundle organizers and legitimate key resellers.
I do not personally use every single supported site. Please let me know if there are problems with script functioning or legitimacy of any particular one.
:(:)The popup window size and other parameters were chosen to fit the activation page's desktop layout on a 1080p screen without scrollbars. If you want a more compact mobile layout, adjust the parameters for yourself as you see fit. I chose not to because there are issues on Steam's end where any error messages in product activation are scrolled off-screen. This (and other issues) could possibly be handled in the userscript but for the time being I've elected not to touch pages originating from Steam. Thanks to @psydex for confirming the v1 series script broke because of a Steam change (and not just for me) and prompting the release of v2.