Is Zelda singing gibberish?

Later in the game you learn the full songs translation from someone in the game

Oh youth, guided by the servant of the goddess, unite earth and sky, and bring light to the land.

Oh youth, show the two whirling sails the way to the Light Tower... and before you a path shall open, and a heavenly song you shall hear.

The longer version of the song is not introduced until later in the game so the part you hear is possibly only part of the first verse.

As to the language from what I've been able to gather its a form of Ancient Hylian (Legend of Zelda series, in world language which is hard to place due to the fact that it is derived from Japanese Hiragana, Katakana, Romaji, and sometimes English in rare circumstances, for both written and vocal forms.

Unfortunately, we don't know. Skyward Sword doesn't really have much voice acting in Hylian, so we just don't have enough data to say whether they actually made a language, or just a translation dictionary, or if it's just the few lines you hear in game declared to mean something specific.

Nonetheless, the answer is probably it's gibberish. (It's definitely not Japanese.)

There are four Hylian scripts. The first two, Old Hylian (Ocarina of Time) and Modern Hylian (Wind Waker), are just fonts for writing Japanese syllables. The second two, the Hylian alphabet (Twilight Princess), and the new Hylian alphabet (Skyward Sword), are just fonts for English.

All the text in the game is written in Japanese or English using these fonts. In Skyward Sword all the signs are written in English, or English-derived gibberish like 'mnop qrstu'. If they had created a language, or even a dictionary, they probably would've written something aside from the song in that language. There would probably also be hints of it in Hyrule Historia, and there are not.