Can someone become certified in
Esperanto? Can someone become C2 (for example) in Esperanto and if so how?
ITK (the Foreign Language Training Center of Eötvös Loránd University in Hungary) does exams at B1 - B2 - C1 levels for those who would like an official certificate from a recognised education authority.
Vanege linked you to a page from where you can find out more about those exams. You can sit them in temporary testing centres around the world, often organised by local Esperantists.
I'm not aware of any official C2 certifications for Esperanto.
Other countries may also have local certifications, but Hungary is a bit special because you can take Esperanto at university level (and supposedly, many Hungarians do so to fulfil the foreign language requirement as "the easy option" if they don't really want a foreign language... some of them stay in the Esperanto sphere, some forget it as soon as they finish their studies).