Even if you don't know everything that's on the menu, you should order something that you can eat.
「メニューに数多いの品が書いてあるが、知らないものは知らなくてもいい、自分の食べられるものを注文すればいい」というのは There are many things written on the menu, and it's ok to not know about some of them. You should just order what you can eat と表現できます。
例文 Don't worry about reading everything that's written on the menu, just order what you want to eat.
「メニューに書いたあるの全部を読むのは心配なくて、ただ食べたいものを注文して。」
参考になれば幸いです。