Final answer:
The language with the letters A, C, G, and U is called the genetic code, which uses codons to translate RNA sequences into proteins.
Step-by-step explanation:
The language formed using just these four letters (A, C, G, and U) is known as the genetic code, specifically the RNA nucleotide bases. Each group of three bases, known as a codon, stands for one amino acid, and the sequence of these amino acids determines the structure of proteins in living organisms.
There are a total of 64 possible codons made from the combination of these four letters, which in turn translate the language of RNA into the language of proteins.