Something like this:
Introduce yourself giving info that is relevant to what you’re asking for. Especially your level of Chinese and English.This is the main thing they need to know about you.
I am an American who has been studying Chinese for 10 years Or
I am a Chinese American.
I am a native Chinese living in China with a good level of English (Proficiency).
I am Hungarian but I have a Chinese boyfriend so I have lived in China for two years. I have been studying English since high school and I’d like to take this opportunity to practice it.
Or whatever else you think fit.
Then what you want:
I have some free time and I would like to volunteer in X show as a Chinese-English subber.
Be sure to include info about whether you can access the show:
I am new to Viki but I have a Vikipass (or Vikipass plus). The show is accessible for my region. I am available for a couple of hours all evenings. OR I will make sure to be available on the days the show is uploaded.
And then some polite formula to finish.
I hope you will consider my request.
Friendly regards/ Sincerely yours/
But do personalise it as you like. It must reflect who you are. And make sure you read it again, checking for typos, grammar mistakes and expression. Because you’re applying for writing English subtitles, so your message will be an example of how well you can write in English.
Surely, there is such a need of native translators (Chi-En and Ko-En) that even people with a medium English are welcomed, as long as the English makes some sense: then the editors can make it correct. But at least the English must be understandable.
How long until you start? I cannot give you an answer. It depends first of all on whether it’s an on-air show or an already completed one. If it’s on-air, work starts when the show starts. If it’s complete, it may start immediately. Or not. Depending on the situation.