I'm looking to do a similar thing and it should be possible but it's not cheap or easy. Check out this paper: https://arxiv.org/abs/2304.08177
These guys did what you intend to do. First they "retrained" the Llama with a corpus of around 20GB of Chinese text, and then they fine-tuned the model on the Chinese translation of the Alpaca dataset. In the first retraining they even increased the vocabulary size of the model.
I'm saying "retrained" because they used LoRA for both the pre-training and for the finetuning, so it's not exactly pre-training as the original model weights are not updated. However, they added enough new weights using LoRA that they achieved an improvement in the understanding of the Chinese language.
Still, I agree with Moiz Husnain in that in your case, this is probably both a harder and inferior approach. Especially if the language in which your chatbot will converse is English.