英语里怎么说古筝？

zither是筝而非古筝.

筝有很多种,中国的,德国 的.....

楼主说的古筝,就是我们中国的筝.
所以可以翻译成 Chinese zither 或者ancient zither

就叫guzheng
The guzheng, or gu zheng (Chinese: 古筝; pinyin: gǔzhēng) or zheng (筝) (gu-, 古 means "ancient") is a traditional Chinese musical instrument. It belongs to the zither family of string instruments.

The guzheng is the parent instrument of the Japanese koto, the Mongolian yatga, the Korean gayageum, and the Vietnamese đàn tranh. The parent instrument of the guzheng is the se.

很多地方直接写Zheng, 或者Guzheng

Zither 古筝

可以用 Chinese Zither 特指是中国的

参考：http://en.wikipedia.org/wiki/Zither

koto

n.

古筝