You should make English as a default language unless if you do really push out for Vietnam users then for sure you wouldnt push this kind of change but if you do plan to go international then
at least consider on make it as a default lang.

Website design is simple and nothing new or surprising and also with that 50USDT bonus, why would really bother it out and make out some confusion?
Why not apply on other coins which doesnt have big fee? Simple.