Interesting that you say Japanese language instead of Japanese music. Personally I don't see anything special with Japanese, compared to any other language (well, maybe it's just that I have nothing with foreign languages in general). For me, Japanese is just something necessary when you are trying to be Zard fan.
Haha its actually my personal experience....Initially i was just contented with listening to ZARD's music and Miss Sakai's vocals but yeah, like you said, learning the japanese language takes the enjoyment of ZARD's works to another level.
Of course there are other artists who can also write great lyrics, while there are definitely other mediums for promoting a language, but ZARD is the inspiration for me picking up Japanese in this case.