Yes, you could make a statement and add a question tag: "You speak Japanese, don't you?" However, in real life, probably a more natural way would be to make an exclamation or expression of pleasure or surprise such as: "I'm impressed that you speak Japanese." It would then be for the speaker to either affirm or disaffirm your belief.
はい、そう言ってクエスチョンマークをつけます。
例文
"You speak Japanese, don't you?"
日本語が話せるのですね？
しかし、実際にはより自然なのは、以下のように感心したり、
喜びや驚きの感情を表す表現でしょう。
例文
"I'm impressed that you speak Japanese.
日本語が話せるのは凄いですね。
その後話している人が、あなたが
言ったことを肯定か否定するでしょう。
"Do you speak Japanese?" This asks them if they speak Japanese.
"Can you speak Japanese?" This asks them to tell you if they speak Japanese.
"I see you can speak Japanese." This will let them confirm if they can speak Japanese once they have spoken it to you first.
例文
"Do you speak Japanese?"
日本語が話せますか？
日本語が話せるかどうか
尋ねています。
"Can you speak Japanese?"
日本語が話せますか？
日本語が話せるかどうか
教えて欲しいということです。
"I see you can speak Japanese."
日本語が話せるみたいですね。
この文は、相手があなたに
日本語で話しかけたので、
日本語が話せるかどうかを
確認しています。
▪Wow, you can speak Japanese?
Wow is am expression to show that you a surprised in a good way.
"you can speak Japanese" is saying and asking at the same time which is a way of also confirming that he/she can speak Japanese.
例文
Wow, you can speak Japanese?
わぁ～、日本語が話せるのですね。
Wowは良い意味で驚きを表す表現です。
”you can speak Japanese”
は尋ねていますが、同時に
その人が日本語が話せることを
確認する1つの方法です。
We can use declarative statements to ask yes-no questions. When such a declarative is written, is it clear that it is a question as the statement has a question mark at the end. However when spoken, one knows it is a question because of the context, and more often because of the intonation used by the speaker.
*I can see you can speak Japanese.
This is way of trying to see if they can actually speak Japanese or they know a few words.
For example:
You:I can see you can speak Japanese.
Teacher: Not really.I can only speak the basic.
例文
*I can see you can speak Japanese.
日本語が話せるみたいですね。
この文は、相手が実際に日本語が
話せるか又は少し言葉を知っている
かどうかを確認しようとしています。
例文
You:I can see you can speak Japanese.
日本語が話せるみたいですね。
Teacher: Not really.I can only speak the basic.
そうでもないです。
基本的な語を知っているだけです。
We can use these three phrases to compliment someone on their level of Japanese despite it being a hard language to learn. We can say, you speak, "very well," or use, "can speak," to how our acknowledgment of their ability. We can also use the phrase, "to be impressed," to show that we are amazing at their level of speaking and understanding.
You can speak Japanese, isn't that right?
Tag questions are questions that can be added to a statement (usually at the end following a comma), which its answer will either be yes or no.
You can speak Japanese, isn't that right?
「あなたは日本語が話せるんですよね？」
付加疑問文は陳述に付け加えることができる疑問文で（大抵は文の最後でコンマに続きます）、それに対する答えは yes か no で答えます。
Get out of here! You speak Japanese like a native.
The actual question and its variants as a question have already been explained here. I would like to offer another possibility based on flattery. If you tell a non-native speaker of English that his/her English is really good, even if you can tell that he/she is a non-native, you will probably make them feel good. After all, learning a foreign language and possibly coming to the country where that language is spoken is a big effort. Obviously, it is not good to exaggerate too much.
If someone compliments on your Japanese/Russian/English, the best answer is "thank you", even if you know that what the speaker is saying isn't true. That way everyone feels good.
実際の質問とそのバリエーションはすでに示されていますが、褒め言葉を含めた他の可能性についても説明しておきたいと思います。
相手がノンネイティヴだと分かっていても、もし英語のノンネイティヴ・スピーカーに英語がとても上手だと伝えると、おそらく相手の気分がよくなることでしょう。
結局、外国語を学ぶことや、その外国語が話されている国に行くことは大変な努力を必要とします。もちろん、褒め言葉をあまり強調しすぎてはいけません。
誰かにあなたの日本語、ロシア語、英語などを褒めてもらったら、それが真実ではないと分かっていても、一番良い返事は Thank you でしょう。
そのようにすると、コメントする側もされる側もみんな気分がよくなります。
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