wo zhengzai hanyu→我正在学习汉语

Wo xuexi Hanyu. Wo shi Baxiren. Wo yao qu Zhongguo xuexi zhongguo de jingjijia.→我正在学习汉语，我是巴西人，我要去中国学习中国经济。

Come on !

wo zhengzai hanyu. 

Wo xuexi Hanyu. Wo shi Baxiren. Wo yao qu Zhongguo xuexi zhongguo de jingjijia.

1).jingjijia（经济学家=jingjixuejia） means economist, you cannot learn an economist. 

you should say: Wo yao qu Zhongguo xuexi zhongguo de jingjixue.

   jingjixue(经济学=economy)

wo xuexi hanyu

Wo zhengzai xuexi Hanyu. Wo shi Baxiren. Wo yao qu Zhongguo xuexi zhongguo de jingjixue.

wo zhengzai xuexi hanyu. 

Wo zai xuexi Hanyu. Wo shi Baxiren.

Wo yao qu Zhongguo xuexi zhongguo de jingjijia.

this sentence have some different mean. You'd better ask me about that when you online.

Hey, Zana. Your words basicly express what you mean. If you take care of some details below, your Mandarin sentences would be perfect.

wo zhengzai xuexi hanyu  ( hanyu is an object here. This sentence need a verb.so I put a xuexi between the object and subject. )

Wo zhengzai xuexi Hanyu.   ( Chinese people would say express it in this way. Zhengzai mean you are learning Hanyu. Or you can write like "Wo xuexi hanyu le" This sentence means you have already started learning Mandarin.)

Wo shi Baxiren. Wo yao qu Zhongguo xuexi zhongguo de jingjijia.  ( Here "xuexi" may make an ambiguity. I'm not sure if you're coming to major economy as a student or you are coming to study on Chinese economy. If you belong to the former, "xuexi" is right. If you are coming to do research on Chinese economy as a journalist or researcher , you may use "yanjiu" instead of "xuexi". At last jingjijia means economist, maybe you want to say jingjixue)

Sorry for my wordiness no matter if it bothered you. If my words confused you, forget about it please.Just follow your feeling and talk more to native speakers. you'll find Oral Mandarin is as easy as pie. I'm learning English and I want to use it as much as I can. So if I made any mistakes please help correct it. Thanks ;).