Actually, this anime doesn't have a "Japanese" style name, it only has an English title, and seems no problem to mark it as Japanese since your media library will automatically fallback to Japanese if it cannot find an English poster :)

Amazon Prime Japan: https://www.amazon.co.jp/dp/B0F2GYZBJV Anime official site(see the top-left corner): https://city-the-animation.com/news/

This is a Japanese anime whose original title is "CITY THE ANIMATION".

As per our rules, posters that include only the original title of the movie/series should always be set to the original language of the movie/series.

Consequently, this poster should be set as Japanese.