Case used with language names

Simple question… I have noticed that whenever languages are discussed in Latin, a case that I can’t label is used. This way, one says “Latine”, “Anglice”, “Graece”, though I have seen at least one instance of “in Latina” (ablative). What is this case used with languages, how is it used, and where else is it used?

If one wanted to say “in old English”, would one say “Anglice antique”… I doubt it, but since adjectives agree in gender, case and number with what they modify, I don’t see why say “no”.

Latinè, anglicè, graecè.

They’re adverbs, ArthurusNoviEboraci, hence with a grave accent on the last letter of each. So “anciently in English (Englishly)” (as a pair of adverbs which don’t themselves inflect) is OK, I believe.
Adverbia sunt, Arthure, ergo accentum gravem habet littera omnium terminans. Deinde, ut opinor, “antiquè anglicè” dicere (ut jugum adverbiorum quae ipsa non inflectuntur) tibi licet.

Could it be the ablative singular of a masculine adjective?

I don’t see why not. You could also add “in” as a preposition to make it clear.

No, because it’s a long ē. There are no adjectives with such a form.

“Numquam ponenda est pluralitas sine necessitate” (Guillielmus de Ockham). Frustrà fit per casum inventum quod potest fieri per formam exstantem.
“Alternatives ought never to be posited if not necessary” (William Occam). No need to invent a new case if a form exists already.

Guillielmus de Ockham was right!! I was getting my vocative mixed up with my ablative! Now that’s embarassing. Any chance I can just disappear that post and pretend like nothing ever happened?!

Anglice, Latine etc. are adverbs. He spoke Latinishly.
‘in Latina’ is short for ‘in lingua Latin’ or ‘in the Latin tongue (language)’.
Hope this helps.