I believe the team has said they are currently evaluating different languages, though they have not divulged which ones. From the FAQ on the Ladybird website:
"We are actively evaluating a number of alternatives and will be adding a mature successor language to the project in the near future. This process is already quite far along, and prototypes exist in multiple languages."
I assume it's either Rust or Jakt (the latter a language in development by the Ladybird lead dev), though I suppose Swift might be on the table.
Creating a new language is almost as insane as building a browser engine. I do not expect this to become a viable browser regardless, but I guarantee it'll fail if they use their own new language for it.
"We are actively evaluating a number of alternatives and will be adding a mature successor language to the project in the near future. This process is already quite far along, and prototypes exist in multiple languages."
I assume it's either Rust or Jakt (the latter a language in development by the Ladybird lead dev), though I suppose Swift might be on the table.