(** Two-letter lowercase ISO-639-1 language codes. The set is deliberately open:
    overlays add languages as data, never as code. *)
type t = private string

val of_string : string -> (t, string) result
val of_string_exn : string -> t
val to_string : t -> string
val compare : t -> t -> int
val equal : t -> t -> bool
val t_of_sexp : Sexplib0.Sexp.t -> t
val sexp_of_t : t -> Sexplib0.Sexp.t