First, proto-languages are not attested. This means that we have no example of writing in any proto-language.
A parent language is typically called “proto-” if the comparative method is our primary evidence about it — i.e. the term is (partially) epistemological metadata.
Proto-Celtic has no direct attestation whatsoever.
Proto-Norse (the parent of Icelandic, Danish, Norwegian, Swedish, etc) is attested, but the written record is pretty scarce, just a few inscriptions.
Proto-Romance (the parent of French, Italian, Spanish, etc) has an extensive written record. More commonly known as “Latin”.
I think the existence of Latin as Proto-Romance has an important epistemological upshot:
Let’s say we want to estimate how accurately we have reconstructed Proto-Celtic. Well, we can apply the same method used to reconstruct Proto-Celtic to reconstructing Proto-Romance. We can evaluate our reconstruction of Proto-Romance using the written record of Latin. This gives us an estimate of how we would evaluate our Proto-Celtic reconstruction if we discovered a written record tomorrow.
A parent language is typically called “proto-” if the comparative method is our primary evidence about it — i.e. the term is (partially) epistemological metadata.
Proto-Celtic has no direct attestation whatsoever.
Proto-Norse (the parent of Icelandic, Danish, Norwegian, Swedish, etc) is attested, but the written record is pretty scarce, just a few inscriptions.
Proto-Romance (the parent of French, Italian, Spanish, etc) has an extensive written record. More commonly known as “Latin”.
I think the existence of Latin as Proto-Romance has an important epistemological upshot:
Let’s say we want to estimate how accurately we have reconstructed Proto-Celtic. Well, we can apply the same method used to reconstruct Proto-Celtic to reconstructing Proto-Romance. We can evaluate our reconstruction of Proto-Romance using the written record of Latin. This gives us an estimate of how we would evaluate our Proto-Celtic reconstruction if we discovered a written record tomorrow.