"She is the only woman in the candidates that/who can speak Japanese."

Neeraj Jain wrote:

What about this sentence? She is the only woman candidate who can speak Japanese.

This is clear, if you want to say that there are other women candiates, but those other women don't speak Japanese. As I keep saying, it's ambiguous without a rewrite, which is why (aside from preferring "who" to "that") I don't find that Marius's alleviates the ambiguity.

The choice between that and who does not change the meaning.
In either case there is ambiguity between whether the relative pronoun (that or who) refers back to woman or candidates.  My first impulse is to take it as woman.

In practical terms, however, it doesn't make any difference.  The same woman is identified as the only one whether she is the only female who speaks Japanese or the only Japanese speaker who is female.

Graphically, it doesn't matter whether we eliminate the non-Japanese speakers first (X row) and then the males, or whether we eliminate the males first (M column) and then the non-Japanese speakers, we still end up in the square which represents female Japanese speakers, and there is only one of those.

               M            F

J                      |     1
             ____________
X                    
                       |

CJ