"Dechreuais i ddysgu Cymraeg achos bod perthynas Cymraeg gyda fi."

Translation: I started learning Welsh because I have a Welsh relative.

[AnthonyCusack]

more a question than anything; isn't Cymraeg the Welsh language and Cymreig the adjective?

[shwmae]

It should be "a Welsh-speaking relative". Cymraeg is the adjective or noun referring to the language. Cymreig is the adjective referring to the noun Cymru (Wales).