Old Welsh is especially hard because the spelling is very different from that of Modern Welsh. Now, Middle Welsh is much closer to literary Modern Welsh

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I mostly agree with Niallbeag on this.

It only really makes sense to write something in a language the reader doesn't understand in very special situations such as when there is an inscription in a language that the protagonist can't understand and perhaps has to decipher, or when the protagonist is thrown into a situation where everyone else is speaking a language that the protagonist doesn't understand and you want to convey the confusion they feel.

The latter device was used to great effect in The Killing Fields where an American who didn't speak the local lingo had to negotiate the chaos of an invasion with a Cambodian friend interpreting important snippets for him sporadically - some people complained that there were no subtitles provided for those scenes but if they had had subtitles the effect would have been lost.

As has been pointed out, you are essentially already translating into modern English whatever language/languages your protagonist actually spoke. I would suggest that you only introduce a foreign language into the English text if your protagonist also can't understand that language (at that point in the story.)

_________________
Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).