Translation:I can speak a little bit of Chinese.
I agree that 汉语 is Mandarin. I too keyed in Mandarin and got it wrong. I reported it. But knowing that the Duo people who are doing this is from Mainland China, I think Mandarin for them is 普通话 bu tong hua.
But to be more specific, 语 "yu" refers to spoken languages whereas 文 "wen" refers to the whole language itself (both written and spoken).
So, when you want to say "I speak..." you use 语 as in 英语 (English), 汉语(Mandarin) etc.
And when you want to say "I am learning...(a language)" you would use 文 as in 中文(Mandarin/Chinese), 英文 (English).
Finally, dialect in Mandarin is 方言 fang yan. So you would use this to refer to Cantonese, Hakka, Hainanese, etc.
Cantonese and Hakka are not dialects of Mandarin. Sichuanese is a dialect of Mandarin. Cantonese, Hakka, and Mandarin are independent Chinese languages. I'm not sure about Hainanese.
Hanyu literally means "language of the Han". The Han are the majority ethnicity of China and speak several languages, each of which has dialects. There are also non-Han languages spoken by the various ethnic minorities, most of which also have their own various dialects.
Yes, but. 會 expresses a learned skill or future occurance. But typically, the future tense use expresses "possibility" or a trend. Your example would more often mean "theres a good chance I will speak chinese sometime in the future" A phrase much more rare than the option allowed here.