While he might be depicted as speaking in the *Eddas*, which are collections of Norse mythology texts, it's important to note that:
* The Eddas were written down in Old Norse. This was a language spoken in Scandinavia during the Viking Age.
* Loki's speech is a literary construct. The authors of the Eddas used Old Norse to create the character and his dialogues.
So, while we can read about Loki speaking in Old Norse, it's more accurate to say he doesn't have a "native" language in the same way real people do.