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paypal populer di Indonesia?

I never hear a word "deman" in Indonesia.. Maybe you mean: "saya bisa membayar 'domain' dengan paypal?"
But the sentence is not right at all, because it is not a question sentence...if you want ask by the sentence, you can say this: "Bisakah saya membayar domain dengan paypal?"

And for "omong-omong" the right is "Ngomong-ngomong(by the way)" :) :)
Nice to help you, friend :)

In my opinion, the sentence "saya bisa membayar 'deman' (demand??) dengan Paypal?" is okay. It's often used in daily conversation with question tone. Yes, it's more formal to say "Bisakah saya membayar 'deman' dengan paypal?" , but your sentence is okay too.
Yes, the correct one is 'ngomong-ngomong', however it sounds 'omong-omong' if you say it fast.

I think paypal is popular in Indonesia, but only among people in big cities..but I don't know for sure.