What does it mean for bitcoin to be legal tender in El Salvador?
Does it mean all shops are forced to accept bitcoin or shops can accept bitcoin only if they want to? Logically, it doesn't make sense for it to be the first option because not all shops will have the infrastructure to (e.g. a small convenience store or grocery shop). If it is the second option, then I doubt many shops will opt for it.
Or does it mean products in El Salador are priced in satoshis?
Keeping it very simple, A legal tender generally means that the Seller has to accept the payment from the buyer in form of cryptocurrencies if he denies payment,
he is not entitled to any payment then. But this generally happens in countries that have just one legal tender particularly their own currency, In the case of El Salvador, they don't have their own currency and have two forms of legal tender, so ideally even if a person rejects payment in BTC but is willing to accept payment in USD, he shall still be entitled to that payment. Because when both of them are legal tenders then it's just based on your convenience.
More importantly legal tender means that if someone pays BTC to the other person, this would be an adequate consideration in the eyes of law, therefore a contract will be established between both parties.