The language of Easter Island belongs to the Polynesian family. The rongorongo are almost certainly written in this language, rather, in its ancient variety. The modern language, however, has been heavily influenced by Tahitian, so much so that the modern name of Easter Island, Rapa Nui, is Tahitian, and that the common greeting, iorana "hello," is also Tahitian (ia ora na).
The dictionary under construction here is an English adaptation of the Ancient Rapanui dictionary included in Sebastian Englert's La Tierra de Hotu Matu'a under the title Diccionario del Antiguo Idioma de la Isla. Englert was careful to identify and warn of Tahitian neologisms and to collect as many archaic terms as he could. Despite the many typographical mistakes, Englert's dictionary is valuable, because it provides a wealth of examples which all appear drawn from a real corpus, part oral traditions and legends, part actual conversations. His dictionary, then, is the best we have for the decipherment of the rongorongo, if they are ever to be deciphered.
The production of this translation was largely automated. You can read about it here if you are curious.
The grammar will reflect Englert's Gramática in the same volume, except for the grammatical model itself, radically different.
The orthography is that used by Englert, except that g will be used for the velar nasal (the sound of ng in sing), which Englert represented by the Greek letter eta, probably because it looks very much like the I.P.A. symbol for the velar nasal, which his printing may not have had. (According to the HTML 4.0 standard, eta is representable by η or η but few browsers recognize it yet).
La Tierra de Hotu Matu'a __ Historia y Etnologia de la Isla de Pascua, Gramática y Diccionario del Antiguo Idioma de la Isla. by Padre Sebastian Englert, O.F.M.Cap. Sixth edition, Editorial Universitaria, Santiago de Chile, 1993. Very difficult to obtain, about US$50 in second-hand bookshops. Try www.biblio.com and www.abebooks.com.