Can I say it?

No. That sounds very awkward and does not actually express gratitude. You could say, "Thanks, it's nice of you to remember my birthday."

That'd be a really odd response. Pretty much the only acceptable response is some form of "thank you".

You can say what you want, but it sounds very strange. :) Are you trying to say it's pleasant that they're wishing you happy birthday? If so, maybe something like "That's very kind/nice of you" would sound more natural.

I think the closest you might be able to get to that phrasing is something along the lines of “That’s so nice of you to say!”, but you’re really better off just saying “Thanks!”

It would come across as awkward and maybe even sarcastic.

I mean no one's stopping you. 

"It's so pleasant" is a bit self-serving without acknowledging that the person congratulating you on your birthday said anything at all. Just say "thanks", or, "yep, another one bites the dust".

"It's so pleasant" would be a great response to someone saying "Lovely weather we are having." (Although that phrase is structured oddly for a new English speaker). As others have mentioned, it would not be an appropriate reply to "Happy birthday".

If you want to let them know you have been having a good day on your birthday, add it after a quick thanks “Thank you, I’ve had a good day”

Are you trying to convey that you are having a good birthday? Then you could say "thank you! It's been a lovely day!" Are you trying to convey that it's "pleasant" that they said happy birthday? Pleasant doesn't quite mean that. "Thank you for remembering" is fine Are you trying to say the *person* is pleasant? Definitely don't say "it" when referring to people. That's for things. Once again "thank you for remembering" is good. As you can see, even when given some context it's an incredibly confusing response. Your best response to saying "it's so pleasant" would be confused silence. When someone says something nice, it's easiest to stick with "thank you" and sound sincere.

It would not pleasant to say that. 

That would be awkward and confusing; they wouldn't know what you meant. And it's rude not to thank them.

No. It is an odd response by itself. You *could* say something like: “It’s so pleasant to see you.” as a way of thanking the person for wishing you happy birthday in person or for coming to your birthday party. Otherwise, simply saying, “Thank you” is the most appropriate response in all social situations. If you also want to express your pleasure at someone telling you that in person, it sounds more natural to say: “Thank you. It’s so great to see you.”

You can say "thanks", "thank you" but I can't think of anything else

No. Completely screwy response. Completely screwy. Off the wall. If you said this to me, I would think "This person is definitely an alien. An alien from Outer Space."

No

To me, the word pleasant is like the word nice. It's lame.

Not necessarily, but I think something close would be “I appreciate that!” if you want an alternative to just thank you. However, in english I’d say not saying the words “Thank you” may be perceived as unappreciative. I’d even combine the two and say “I appreciate that, thank you.”

It does sound odd the way you have it. One way you could still use “pleasant” would be to phrase it: “Thank you, it’s been so pleasant!” This both thanks the person and ensures them that you have been experiencing a pleasant birthday thus far.

You could say something such as "Thank you, I'm enjoying the day!" if you are trying to get across that your birthday has been pleasant.

Is that a literal translation of something they say in your language to thank?  In English it doesn’t really make sense in that context 

As others have said the most natural response is ´thank you’; perhaps the closest idiomatic phrasing that matches the sentiment you seem to be expressing might be ‘how kind of you!’ or simply ´how kind!’ But I would still thank the person :)

What is pleasant? Why would anyone respond like this?

It’s a really awkward and unusual thing to say, but personally I would find it cute (even if it doesn’t make a lot of sense)

Why would you want to do that?

I think the idea you're trying to convey is something like: "Thanks, I'm having a great day!"

You are probably a Russian speaker) We wouldn’t say this. You can just say thank you very much!

No, because it doesn't make sense. The person you say that to will go 'huh?'

I think what you're trying to say is something like "that's so kind/sweet/nice of you", but usually I would also follow that up with "thank you".

I don't think people would know what you meant. It would sound positive but they'd definitely ask, "What?"

No that would make you come off as a not a native speaker

No. You could say "That's so nice of you!" but even that would still be odd to some speakers

"it's so pleasant" is a phrase indicating that an object or idea is giving you a feeling of enjoyment. It is not an expression of gratitude to the individual that said that to you in the slightest. Complete side bar: if you meant to refer to the person with" it", never do that. Some people will just be confused, but others will be VERY offended.

You would definitely get a confused look if you said that. I’ve never heard anyone express gratitude in that way. I would stick to “thank you.”

No. That would be a very strange response.

No, "It's so pleasant" does not work in that situation at all. If you said that to a person wishing you happy birthday, they might think you misunderstood them and you are replying to what you thought you heard. They might say, "Happy birthday" again, louder and with exaggerated enunciation to make sure you understood them correctly. Then everybody's confused. 90% of the time, just respond with "Thank you." You could say "I appreciate it" or "Much appreciated," but "Thank you" is the best response.

You could, but most native English speakers would think you a bit weird for doing so. However, grammatically, it's fine.