Translation:– Why do you never speak English? – I am so shy.
"I am so shy" may be a literal translation, but it is a very unnatural answer to this question in English. A more natural answer would be "I am too shy" - as in, not "more than very shy", but "too shy to speak it". Perhaps "quite shy".
Can't explain very well why, but "so" just doesn't work here, it would only have worked if this was just a statement of fact.