Here's why:
* Subject-verb agreement: The subject "I" is singular, so the verb should be "sing," not "sings."
* "In tongues" is a specific phrase referring to a religious practice, often associated with Pentecostalism, where people speak in an unknown language.
Here are some correct ways to say it:
* "I speak in tongues."
* "I sing in tongues." (This is acceptable if you are specifically referring to singing in the "unknown language" associated with the practice.)
* "I speak in an unknown language."
Choose the phrase that best fits the context you are using it in.