I agree with the other two. I think は sounds a lot nicer.

Both is right Both case is Japanese is subject and Answering case of Qustion 日本語がよくわかりません　 answer for question of what language do you not understand? . 日本語がよくわかります　. Answer of question of what language do you understand? in the conversation gernal taliking with Japanese you use '”は”　is smooth. In this case Like a Quetion term when you answer question that case should be subject of sentance make clear that case "が”　is effect to Japanese

僕もrjcorrayaさんと同じで、どちらも正しいと思いますが、「〜が」より「〜は」の方がいいと思います！

I think both of them is correct. However, I think you need to use ～は instead of ～が unless you have any other intension of using it. I hope other native speakers give you better answers. ^_^