When reconstructing a language it is necessary to have a corpus of literature, but how can literature be translated in the first place without a reconstructed grammar and lexicon?
Reconstruction is considered to be a serial process comprising (i) transcription, (ii) translation, (iii) analysis, and (iv) modelling; although to some extent this process is iterative and it is often necessary to return to earlier steps to complete the process.
The problem of the reconstruction of Brittonic is central to providing accurate translations of Early Cornish literature and epigraphs since there is no remaining native body of speakers. Documents and monuments are the raw stuff of reconstruction, but without an accurate and consistent model of Brittonic translation quality will be poor or incomplete, and most likely based on assumptions depending on the language skills of the translator (who may be primarily concerned with Latin, Welsh, Breton or less likely, Cornish).
However, reconstruction activities depend on the quality of transcription of originals, and transcription usually involves some knowledge of the source language, so it is not an activity truly independent of translation. To perform a correct translation, translators need enough of the text, in un-garbled form that makes grammatical sense. They also need some knowledge of the language concerned, not in its present form, but from the period being studied.
This is the chicken-and-egg problem.