Overall, working in an European country was a great and eye-opening experience.

I could alswas able to learn many cultural differences.

It was also very interesting to know that there were also huge differences between German and American cultures.

As a person who lived in a small island for a long time, the experience of having lived in the US was already an astonishing experience and sometimes undermined my view of many things.

There might be a better word to use other than "undermined" to convey your message.

The experiences living in different countries greatly helped me realize the importance to learn, respect, and get accustomed with each country's culture, being humble without having any prejudice.

After my 2two year-longs working in Germany, I went back to Japan.

Changed to some more natural phrasing.

You could also say "2 year long period of work in Germany" which is closer to your original version, but I think the corrected version is most natural

I could also learn(learned / could learn) (about) many cultural differences.

Removed "also" because "also" is for adding another point but this seems to be more about expanding the previous point.

"about" is optional here, but I think it sounds more natural to include it

"could learn" isn't wrong, but I think "learned" is more natural since you're talking about an event in the past so you know the outcome.

It was also very interesting to know that there weare also huge differences between German and American cultures.

Even though you learned about them in the past, these differences still exist, so the differences are talked about in the present tense.

As a person who lived in a small island for a long time, the experience of having lived in the US was already an astonishing experience and sometimes undermined my view of many things.

Just a heads up "undermined" has a negative connotation

Sometimes it felt challenging to live and work there because a lot of experiences I gained in the US didn't work and the cultural norm was very different from either of Japan andor the US.

I used to work for an American company, and I now work for a German company, so I got to experience this recently too!

It still feels embarrassing, but for the first month in Germany, I had a prejudiceexpectation that while each Western county has their own history and culture, their culture and norms should have a lot in common.

I think "prejudice" is probably too strongly negative a word for the point you're trying to make here.

The experiences living in different countries greatly helped me realize the importance tof learning, respecting and getting accustomed with each country's culture, and being humble without having any prejudice.

I am very grateful for all those experiences and hope these experiences could bringlead to further fruitful experiences.

Overall, working in an European country was a great eye-opening experience.

I could also learned many cultural differences.

As a person who lived in a small island for a long time, the experience of having lived in the US was already an astonishing experience and sometimes undermined my view of many things.

Sometimes it felt challenging to live and work there because a lot of experiences I gained in the US didn't work and the cultural norm was very different from either ofboth Japan and the US.

The experiences living in different countries greatly helped me realize the importance tof learning, respecting and getting accustomed with each country's culture, and of being humble without having any prejudice.

In turn, I also found myself more respectful and mindful of the Japanese culture.

I am very grateful for all those experiences and hope the experiences couldwill bring further fruitful experiences.