Stack Overflow in Portuguese has certain development-related questions, but they are not about programming itself, due to the fact that Portuguese stacks are still very recent in relation to stacks in general. In English, you can ask about
Graphic Design . There is even a question similar to yours:
How do I know if my downloaded font is legal
And the answer is that every case is a case. You should research the individual license of each source. Unfortunately this is a job you should do for each different source you use. But maybe you can find some cured list for the most commonly used fonts.
Each author publishes their sources with a different license. In general, anyone who uses Creative Commons releases them for non-commercial use without payment, but for commercial use you may have to pay a license.
Now ... Sans-serif is not a source, it's a feature that several sources have in common. Sans-serif means "without serifs , which are a visual markup that makes it easy to read some people.
Several fonts have Sans-serif in the name, to make it clear that they do not have serifs, but that alone does not define the name. Find out the name of your source and check how its authors make it available for free or commercial use.
