早く飲まないと氷が溶けて味が薄くなるよって英語でなんて言うの？

water down' (verb) and 'watered down' (adjective) are the phrases we use to refer to the lack of strong flavor a drink has after ice melted into it. When the ice melts into water, it takes away from the drink's good flavor, so we call it 'watered down'.
For example:
"I don't want my orange juice anymore. It's watered down." (adj.)
"I don't want ice in my tea because it will water down the flavor after awhile." (verb)

I would suggest that you drink faster before the ice melts and over dilutes your drink.
- This means you would like them to drink faster because the ice will melt and affect the taste of their drink.
Dilute is a verb that means to make a liquid weaker by adding another liquid.
For example:"Bleach can be diluted with cold water."

>You better drink faster, the ice is about melt and the juice will be watery.
*This is telling your friend that he she must drink quick other the ice will melt than it will taste like water.

>If you want to enjoy your drink, you better finish it before the ice melts.
*This is telling your friend that if he/she want to enjoy the drink than they must drink a little faster.

To dilute = to make something weaker by adding something else.
Yes, in a hot country when you are dying of thirst and finally get your hands on an ice cold drink, you want to savour the wonderful chilled flavour at its best!
"Wow, this cola tastes just amazing!"
"If you don't want a really watery drink, you'll need to down that quickly!"
"I really don't care! As long as it's cold, it's wonderful. My priority is to cool down."

When you have ice in your drink as soon as it starts to melt it makes the drink lose flavour as it 'dilutes' it or 'waters it down' this means there more ice water in the drink as it melts
Diluting is when something has water added to lose it flavour or make the taste weaker/not so strong