In this paper we outline some phenomena where Formal Language Theory (FLT) and First Order Logic (FOL) meet and open new perspectives in both their investigation fields. What will turn out to be essential, in this direction, are the notions of string model and string theory that are defined by extending the linear and concatenative structure of strings to FOL terms. Many examples are provided of logical representation of languages. The internal structure of formalisms in FLT is logically described by means of string derivation systems. Finally, an interplay between logical representability and computational power is showed in Cook-Karp’s theorem of NP completeness of propositional satisfiability and in Godel’s incompleteness theorem.