The Vietnamese language is being mispronounced (all models)

### Self Checks - [x] I have thoroughly reviewed the project documentation (installation, training, inference) but couldn't find any relevant information that meets my needs. [English](https://speech.fish.audio/) [中文](https://speech.fish.audio/zh/) [日本語](https://speech.fish.audio/ja/) [Portuguese (Brazil)](https://speech.fish.audio/pt/) - [x] I have searched for existing issues [search for existing issues]([https://github.com/langgenius/dify/issues](https://github.com/fishaudio/fish-speech/issues)), including closed ones. - [x] I confirm that I am using English to submit this report (我已阅读并同意 [Language Policy](https://github.com/fishaudio/fish-speech/issues/515)). - [x] [FOR CHINESE USERS] 请务必使用英文提交 Issue，否则会被关闭。谢谢！:） - [x] Please do not modify this template :) and fill in all the required fields. ### 1. Is this request related to a challenge you're experiencing? Tell us your story. Hi team, I'm having trouble with Vietnamese language support. It seems to be the only language underperforming—I've tested 50+ voices and personal clones, but the pronunciation is consistently off. I suspect it might be a model-level issue. ### 2. What is your suggested solution? I tried self-hosting and fine-tuning to fix this, but the volume keeps dropping significantly after training. Could you please look into this? The Vietnamese community is quite active and would really benefit from a fix! ### 3. Additional context or comments _No response_ ### 4. Can you help us with this feature? - [ ] I am interested in contributing to this feature.