Whether the English translation should use simple present or present continuous is determined by the meaning, it mostly cannot be derived from the Czech verb form. Exceptions are certain verb pairs such as chodit / jít.
Here, since we seem to be talking about all Fridays, simple present must be used.
Fun fact: this probably refers to the Good Friday, called Velký pátek (Great Friday) in Czech (more on http://www.myczechrepublic.com/czech_culture/czech_holidays/easter/good_friday.html )
Like was said before it should be we eat fish on fridays not friday. if it means every Friday. for only one Friday then it should be this Friday or we are eating fish on Friday /this Friday. But i don't really care. I do want to know though what would be the Czech Translation for we eat fish on Fridays or every Friday.
Is wrong English for you? Good to know so we can consider your other claims in the appropriate context.
Yes, and it's the same in Czech. It's fine to say "v pátek" (singular) even when it's habitual and it happens every Friday (or on multiple Fridays).
Alternatively, we can say "každý pátek" (every Friday).
Using the plural requires a preposition: "o pátcích" (on Fridays), but it's not used much nowadays.