You must know either Finnish or Swedish. One requirement for becoming a Finnish citizen is that you have at least satisfactory skills in Finnish or Swedish. There are many ways to prove your language skills. You can present a leaving certificate issued by a Finnish comprehensive school or a Finnish upper secondary school. Alternatively, you can complete the National Certificate of Language Proficiency test at level three or higher. Exceptions to the language skills requirement can be made but this happens only in very rare cases.
The language skill requirement only applies to applicants who cannot obtain Finnish citizenship by declaration. You can obtain Finnish citizenship by declaration if you are, for instance, a former Finnish citizen.
Finnish Immigration Service