Is it a good idea to diversify this way ?

Let's say I have some big amount of cold money, is it a good idea to open fixed deposit account at 4 banks each in around 25 countries.

Is it a good idea ? And would I need to pay tax in the interest earned, supposing I let it compound in it's account.

Thanks

No and it depends on the country.

In most (all?) countries, you will be losing ground to inflation with bank deposits.

What is your goal with this money?

Just put that in bonds or something similar.