素材の味が分からなくなる って英語でなんて言うの？

The first example is the preferred way of telling someone that what they're doing is having a negative effect.

The second example would likely result in the person you are speaking to feeling ashamed and offended.
Only use the second one if you are certain that the person you are speaking to will take it as a friendly joke and won't be offended or if you do not want to be their friend anymore anyways. The first example is a far better choice.

In order to explain adding too much of a condiment (a condiment is a sauce, for example soy sauce, tomato ketchup or salad cream) will disguise the taste, you can say:

You won't be able to taste anything with all that soy sauce.
This is an informal way of saying that with the amount of soy sauce the person is using, they will not be able to taste anything (in this case the ingredients in the sushi)

You can also say:
The soy sauce will mask the taste of the sushi/flavour.
To mask - To cover, to hide, to disguise, to cover with a mask.

I hope that helps!

'To drown the food with _' We use this expression when a lot of sauce or liquid such as gravy or custard is added to a dish, whether it be a dessert or a savoury main course.
"You are drowning your Victoria pudding with custard - it will spill over the bowl!"
"You are drowning those french fries in tomato sauce: you'll only be able to taste ketchup!"

To say that something will "overpower" the taste of the dish will let them know that the flavour of the sauce will be too intense or strong to taste the rest of the meal.

Or you can explain to them that the dish is supposed to be eaten with a small amount of soy sauce, in order to experience the dish the best way.

If something has a subtle flavor it means that it is not very strong.
Subtle is pronounced like ''suttle''.

A common phrase is also to ''drown'' you food in something or for it to be ''swimming'' in it.
Examples:
Don't drown the sushi in soy sauce, you won't be able to taste the sushi.
The sushi is swimming in soy sauce!

If you want to be polite then you would say 'all that soy sauce might mask the flavour of the sushi'
To mask something means to hide it so it wont be as obvious or as tasty
The other 2 are still polite but might come across as slightly ruder depending on who you are eating with (as everyone has their own opinon and likes things different)