Sleazy Fork is available in English.
Extension for Manga OnlineViewer Fluid Mode + for 18+ sites, allowing double-page spread viewing for halved images. Sites: Fakku, HBrowse, doujin-moe, hentai2read, luscious, wondersluts,hentaimangaonline, exhentai/g.e-hentai, pururin, hentai4manga, doujinshihentai, hitomi, nhentai, hentaihere, 8muses, Tsumino etc.
Manga OnlineViewer Fluid Mode + is Dead
I couldn't get the viewer to work on most sites, and I had to manually add URLs in order to get it to work for the ones that it still does. The script was last updated in 2015, so I think it's safe to assume that this script is dead.