會講國語 vs. 講國語: when to use one or the other?
我會講國語 means you are able to speak Mandarin(probably be good at it), which is often used when explaining to someone if you can speak Mandarin. You might put 我可以講(一點)國語, in case that you might not be very good at it. 

Compared with 我會講國語, 我講國語 doesn't imply whether you are good at it or not. 

我講國語 is often used when asked what type of/how many languages you use or speak. For example,

A: 你講幾種語言 ?

B: 我講國語，英語，...

... ...

A: 你講什麼語言 ?

B: 我講國語

A side note: in mainland, we say 普通话 or 中文 for Mandarin. I heard TWN folks said 國語, and maybe HK folks as well.