"There are many things I do not know."
「（私が）知らないことが多い」できるだけ直接翻訳したものです。"many things"は「たくさん。のこと」で、"know"は「知っている」に相当します。
"There are countless things I do not know."
「知らないことが数えきれないほどある」強調したいなら、"countless"、つまり「数えきれない」、という単語も使えます。
参考になれば幸いです。
There are a lot of things that I don't know about Japanese culture.
「知らないことが多い。」という文章を英語で表すと「There are a lot of things that I don’t know.」という文章を使っても良いと考えました。「知らないこと」は「things that I don’t know」という表現になって、「多い」は「a lot」という言葉になります。日本の文化について話す場合、「There are a lot of things that I don’t know about Japanese culture.」という表現を使っても良いです。「について」は「about」になります。
Though I am Japanese, there are still many things I don't know about my own culture.
There are a lot of things I still don't know about my own culture.
There are many things I've yet to learn about my culture.
When a person has had a long time to learn something but never learned it, we usually use the word 'still' for exaggeration. For example, "I still don't know many things about my culture." This means that you've been learning for awhile but never fully learned everything. It is also common (and formal) to use the phrase 'have yet to...' to refer to something you have not done yet.
何かを長い間学んでいるのに学び切れていないことを表すとき、誇張する言い方として 'still' がよく使われます。
例えば：
"I still don't know many things about my culture."（まだ自分の国の文化について知らないことがたくさんあります）
これは、しばらく学んでいるけど学び切れていないという意味です。
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また、'have yet to...' も、まだしていないこと表すときよく使われる（フォーマルな）言い方です。
Even though I'm Japanese, I'm still learning about Japanese culture.
"Even though" creates a contrast between the fact that you are Japanese but still don't know a lot about your culture. Sometimes it's embarrassing to admit that you don't know something, so instead of that, you can say that you are still learning. I hope that this helps. :)
"Even though" は、日本人であることと、日本の文化をあまり知らないこととを対比させています。
何かを知らないことを認めるのはばつが悪いこともあると思います。ですからその代わりに、"I'm still learning"（まだ勉強中です）と言えます。
参考になれば幸いです！
There's loads about Japanese culture I have yet to understand even though I am Japanese.
There's a lot I don't know about Japanese culture, despite being Japanese.
If you want to say that even though you are Japanese, there are still many things that you don't know about Japanese culture, you may speak in terms of either 'understannding' or 'knowing'.
The English expression "Not well aware" is used for saying that you are almost, but not completely certain of something. In your situation you could use this expression because although you know Japanese culture, you are still not completely certain of all the aspects of Japanese culture.
"Not well aware" は、完璧ではないもののよく知っていることに使われます。あなたの状況では、日本の文化を知っているけどもその全てに精通しているわけではないということなので、この表現が使えます。
Even though I am Japanese, there are certain things I don't know about the Japanese culture
I don't know everything about the Japanese culture even though I'm Japanese
When you want to explain that you still do not know many things about the Japanese culture even though you are Japanese, then you may explain this in the following ways:
-Even though I am Japanese, there are certain things I don't know about the Japanese culture
-I don't know everything about the Japanese culture even though I'm Japanese
「日本人だけど日本の文化についてあまり知らない」は次のように言えます。
-Even though I am Japanese, there are certain things I don't know about the Japanese culture
（私は日本人ですが、日本の文化について知らないこともあります）
-I don't know everything about the Japanese culture even though I'm Japanese
（私は日本人ですが、日本の文化について全てを知っているわけではありません）
ブルガリア 
アメリカ合衆国 
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