In the Unfinished Tales it is mentioned that Sindarin and Silvan elves living in Mirkwood were mostly illiterate. Therefore, Oropher, Thranduil and Legolas did not know how to read or write in Tengwar script. And then it just clicked. That is the reason Legolas doesn’t say anything about the text at the gates of Moria in FOTR. Why? Because even through it is in elvish, it is written in Feanorian characters, Tengwar. So then everyone is staring at Legolas, who is probably mortified because nobody is supposed to know that he and his father, the great elvenking, are illiterate. Gandalf knows and so he just reads the script, trying not to laugh at his horrified expression.