Hello Toots and Happy New Year!

First of all, Japanese people tend to pronounce English in their own way (like people from other country, by the way, we're all influenced by our mother tongue). As for "San Kyuu", the sound [k] being attached with the sound [yu], there is also another reason: it's because of the reading of the numbers 3 (san) and 9 (kyuu). A few years ago, people would write "39" (san kyuu) to say "Thank you".

I hope this answer your question!
I think its almost the same with Chinese, they would use numbers to represent some words for example, they will write '3Q' in social chats meaning 'thank you' 3 in chinese is 'san' and 'q' sounds like 'you'
Thanks suyi01, it's always interesting to compare with other languages.

In Japanese, there are so many words plays with numbers, and it's sometimes quite hard to guess because numbers in Japanese can have a Japanese reading, a Chinese reading or an old Japanese reading. This one: "4649", meaning "yo-ro-shi-ku".

I guess Chinese people may have the same kind of words plays.