How would you define "twice as many" when 0 is concerned? Consider the statement that "$\mathscr{L}$ is defined as a language where all strings contained in it contain twice as many b's as a's." Clearly a string containing a single a will yield two b's, and so on.
The issue that I'm having with a statement like this is what happens if there is a string that contains no a's?
Mathematically for something to be "twice as many" would be defined as y is twice as many as x given that $y = 2x$. It is true that $0 = 2\cdot0$ is a true statement, however I have trouble seeing if we would define then 0 as being twice as much as zero. If the definition of twice that I posted above is true, then 0 would be defined as twice as much as 0, however that seems to not be what the English word actually has in mind. How can nothing be twice as much as nothing?
Am I misunderstanding the phrasing of the original question? If there are twice as many b's as a's, must there be at least a single a for the statement to make sense?
 A: No a's would mean no b's.  When you're dealing with math, usually statements written in informal mathematical English are meant be interpreted using the obvious translation as formalized mathematics.
For example, we might write the quoted statement as
$$\mathscr{L} = \{ s\in \mathbf{strings} \mid \mathbin{\#}_{\mathbf{b}}(s)=2\mathbin{\#}_{\mathbf{a}}(s)\}$$
where I've made up some notation since I'm not sure the conventions: $\mathbf{strings}$ is the set of all strings, and $\mathbin{\#}_{\mathbf{b}}(s)$ is the number of times b appears in the string $s$.  Languages are defined to be subsets of $\mathbf{strings}$, which is why I've written it as a set.
If you wanted to exclude zero b's, the statement would have likely been written like "$\mathscr{L}$ is defined as a language where all strings contained in it contain at least one a and twice as many b's as a's."  This is (more-or-less) because you need this additional "$\mathbin{\#}_{\mathbf{a}}(s)>0$" in the formalized version.  There aren't any hard-and-fast rules for informal mathematical English, though, but this is at least a good way to write so you're understood.
Another example of a place where English diverges from informal mathematical English is with "or."  If I said "the string contains an a or a b," then it is permitted to contain both.  Compare to "you may take an apple or a banana as a snack."
