"she is the only first person WHO" is incorrect. You can't combine "only first" like that.* Pick one of three, like:
"She's the only person ever to have graduated from Harvard at age 7."
"She's the first person ever to graduate from Harvard at age 7."
Or, if need be,
"She's the first AND only person ever to have graduated from Harvard at age 7."
So, in the topic starter's example, if you wanted to use both 'first' and 'only', you'd say something like:
"She is the first and only person who can speak seven languages."
* Only in special cases you can, where it means something else, like: "Only first and last name, please."