If you come to Japan, chances are you'll have a conversation like this at some point:

You:
こんにちは。
Hello.

Japanese Person:
わあ、日本語お上手ですねえ！
わあ、　にほんご　おじょうず　です　ねえ！
Wow, you're so good at Japanese!
Literally: "wow, + Japanese + skilled + is + ねえ！"
Note: You might also hear it without the honorific お before 上手 and without the わあ or the extra emphatic え on ねえ.

Sometimes it feels like you could mumble gibberish that halfway sounds like Japanese, and people would still tell you this.

My guess is that some of them actually think we're good at Japanese, while the rest are just flattering us.

That's nice. Whatever.

More important is this problem:

💀 How should you respond when someone says this to you? 💀

Our options are pretty limited for this one, but I recommend any of the following, which I'll split into casual and polite situations...

🍻 Casual Responses 🍻

Japanese Person:
日本語上手だね。
にほんご　じょうず　だ　ね。
You're good at Japanese.

You, Option #1:
ありがとう！
Thanks!

Since it sounds pretty confident, this nice, simple option is only recommended if you really are in fact good at Japanese. If you think that you still suck, then you might want to go with one of the next two options...

You, Option #2:
ありがとう！まだまだだけどね。
ありがとう！　まだまだ　だけど　ね。
Thanks! I'm still far from good, though.

まだ means "still; yet," and when you repeat it, you get まだまだ which in this case sort of strengthens the meaning of "(not) yet," which, in turn, gives it the nuance of "still having a long way to go." In other words, you are still far from good at Japanese.

You, Option #3:
全然上手じゃないよ～（でもありがとう。）
ぜんぜん　じょうず　じゃない　よ～　（でも　ありがとう。）
I'm not good at all! (But thanks.)
Literally: "not at all + skilled + am not + よ～ + (but + thanks.)"
Note: We studied 全然 back in Lesson #44.

I think I'm going to try this one the next time a friend (or a friend of a friend, more likely) tells me that I'm good at Japanese. Because in the mind of a dedicated language learner, one always has so much further to go, you know? My Japanese will (perhaps) never be perfect, but that means that I can continue to grow indefinitely, and there are few things as rewarding in life as continuous personal growth.

You, Option #4:
ありがとう！頑張って勉強してるんだ。
ありがとう！　がんばって　べんきょう　してる　んだ。
Thanks! I try to study a lot.

I kind of had a hard time saying this one, because 頑張って just means "try one's best" or "work hard." Again, I wouldn't answer with something like this unless you're pretty confident in your Japanese level.

👔 Polite Responses 👔

Japanese Person:
日本語上手ですね。
にほんご　じょうず　です　ね。
You're good at Japanese.

You, Option #1:
まだまだですよ。
まだまだ　です　よ。
I'm still far from good.

This is the same まだまだ we saw earlier, only we have です because we're being polite. When I attended a Japanese language school way back in the day, this is what they taught everyone to say... so of course I got tired of saying it (and hearing all of my gaijin friends say it) after about a week.

You, Option #2:
とんでもないです。（まだまだです。）
とんでもない　です。　（まだまだ　です。）
Not at all. (I'm still far from good.)
Literally: "outrageous + is. + (far from it + am.)"

This one is my favorite! I only use it when speaking politely, but it always seems to impress Japanese people, as it's a pretty unexpected--and highly polite--thing to say. One time a girl working at a coffee shop in Shinjuku told me that I was good at Japanese. I came back with とんでもないです, and she said something along the lines of, "The fact that you responded with とんでもないです shows that you are good at Japanese." That was cool. ^_^

You, Option #3:
ありがとうございます。
Thank you.

Confident. Polite. Succinct.

You, Option #4:
ありがとうございます。頑張って勉強してます。
ありがとうございます。　がんばって　べんきょう　してます。
Thank you. I try to study a lot.

This is the same as the one we saw earlier, but a bit more polite.

Anyways, today's lesson was a bit short, but I can 99.999% guarantee that you will need these Japanese phrases. Especially as you continue to get more and more お上手.