You could argue (as both the English and Esperanto Wikipedias do) that the main inspiration for Newspeak was Basic English, which Orwell had been a fan of before rejecting. The minimal vocabulary premiss of Basic English (revisited in xkcd: Up Goer Five) is something Orwell derides in Newspeak.
But minimal vocabulary was also a selling point of Esperanto (though one that has been watered down in practice). And the morphology of Newspeak is pretty clearly an allusion of Esperanto and not Basic English. Behind doubleplusungood, you can see Orwell wincing about Esperantists’ praise for compounds like malbonega.
There is excellent circumstantial evidence behind this, which is recounted in the Esperanto Wikipedia article. (See also What is Esperanto? And why did it irritate George Orwell? and George Orwell et l’espéranto)
Orwell himself certainly knew about Esperanto. He went to Paris in 1927 to improve his command of French and to visit his aunt Kate Limouzin, who was the partner of the founder of SAT, Eugene Lanti. Esperanto was the main language used in the house, and for Lanti, as Orwell found out, Esperanto was not just a language but also an ideology. Lanti demonstrated to Orwell (if he hadn’t already realised it) the connection between politics and language. Orwell suffered, because he did not speak Esperanto.
Orwell in Burma, just before going to Paris. I gather those kinds of moustache were all the rage at the time.
And Eugène Lanti wasn’t just some random Esperantist.
Eŭgeno Lanti. Ne nur hazarda esperantisto.
His nom de plume L’anti “The anti-guy” was befitting a socialist opponent of Stalinism, who launched his own anti-nationalist ideology within Esperanto (Anationalism), and who ran the Sennacieca Asocio Tutmonda. SAT between the wars was the socialist alternative to the “bourgeois” Universal Esperanto Association, and its membership was huge.
Now. You’re 24 and Down and Out in Paris and London, crashing in your aunt’s place, and relying on her handouts. Your aunt is shacked up with a nutjob socialist who keeps preaching about Esperanto. You came to Paris to learn French, and all you hear the livelong day is Esperanto this and Esperanto that, and Esperanto proletarian victory this, and Esperanto against nationalism that—and all of it IN goddamn Esperanto.
Twenty years later, wouldn’t you want to take just a little revenge?