The Hylian Language is the language spoken by the Hylian as well as the primary language of the Kingdom of Hyrule in the Zelda series. Despite some other Hyrulean people having their own language like the Gerudo and the Sheikah, they still mainly use Hylian as a way to communicate in the games as almost everyone in the kingdom speaks it. Given the enormous time scale between the first and last game of the timeline, the script of the language and the language itself changes a lot depending on where you are in the Timeline.
Six variants of the language currently exist. The name given to the scripts by the community is based on release order and not chronological order, which means that the First Hylian Script is not the first instance of a Hylian dialect in the Timeline, just the first instance of said dialect in the history of the series.
First Hylian Script
The First Hylian Script is the name given to the script found in The Legend of Zelda: A Link to the Past. Unlike the other scripts, it is utterly nonsensical and is composed of three symbols (six in the Japanese version) that are arranged randomly in strings. Link needs the Book of Mudora to understand it as it is supposedly a dead version of the language by the time of A Link to the Past.
Second Hylian Script
The Second Hylian Script is the name of the language found on various signs and architecture in The Legend of Zelda: Ocarina of Time and The Legend of Zelda: Majora's Mask. It is actually a cipher of the Japanese Kana where certain letters represent entire syllables. It actually lacks certain characters that are used for contraction and punctuation. It is not an accurate cipher and makes it difficult to perfectly emulate actual Japanese text.
Third Hylian Script
The Third Hylian Script is the name of the main language found in The Legend of Zelda: The Wind Waker, The Legend of Zelda: Four Swords Adventures, The Legend of Zelda: The Minish Cap, The Legend of Zelda: Phantom Hourglass and The Legend of Zelda: Spirit Tracks. It also makes a small cameo in The Legend of Zelda: Twilight Princess, which uses the Fourth Hylian Script as its main language.
Third Hylian makes little sense chronologuically as it is spoken during the Force Era before being replaced by Second Hylian during the Era of the Hero of Time and inexplicably becoming once again the main spoken language during the Era Without a Hero before dying out by the time of The Wind Waker where it is only spoken by beings that were alive before the Great Flood. It is also spoken in the World of the Ocean King and is once again revived by Tetra and her settlers when they found New Hyrule, making it the official language once more. It also is inexplicably revived and succeeds Fourth Hylian in the Era of Shadow.
Third Hylian is also a Japanese cipher like Second Hylian but is way more complete than the later and can transcribe Japanese sentences more accurately. However, Third Hylian lacks Yōon compatibility, as all letters are full form, lacking minuscule form diphthong lettering.
Fourth Hylian Script
Fourth Hylian Script is the name of the language spoken in Twilight Princess, which also makes a brief cameo in The Legend of Zelda: Skyward Sword. Unlike the scripts that came before it, Fourth Hylian is not a Japanese cipher but a Latin one. Fourth Hylian can be used to transcribe most Latin based languages but it is specifically based on the english alphabet and every Hylian words found in this script can be directly translated to english as it is the language used as a base to write every text in Fourth Hylian in the games.
Fifth Hylian Script
The Fifth Hylian Script is the language spoken in Skyward Sword, it is technically the oldest form of Hylian in the whole series. It was replaced by the Third Hylian Script in the Force era. Unlike almost all other scripts, the Fifth Hylian Script never resurfaced in a later era. Like the Fourth Hylian Script, it is a Latin cipher based on the english language. Unlike Fourth Hylian, certain characters are duplicate: D is identical to W, F is identical to R, J is identical to T and O is identical to Z.
Sixth Hylian Script
The Sixth Hylian Script is the language spoken in The Legend of Zelda: A Link Between Worlds and The Legend of Zelda: Breath of the Wild. It is the latest known Hylian language spoken in the Timeline, still existing over 10,000 years after every games that aren't after Breath of the Wild. Sixth Hylian is once again a Latin cipher that is based on the english language. Like Fifth Hylian it has duplicate characters: D is identical to G, E is identical to W, F is identical to R, J is identical to T and O is identical to Z.