YOU SAID:

I have dirt in my fingernails

INTO JAPANESE

私は私の指の爪に汚れがあります

BACK INTO ENGLISH

I have dirt on my fingernail

INTO JAPANESE

私の爪に汚れがあります

BACK INTO ENGLISH

My nails have dirt

INTO JAPANESE

私の爪は汚れています

BACK INTO ENGLISH

My nails are dirty

INTO JAPANESE

私の爪が汚れている

BACK INTO ENGLISH

My nails are dirty