It's too long for that. European words meaning light and bright are stems - only one syllable plus the declension (usually dropped - virtually none remain in English). If Illyana is a variant of Greek
Helene it may mean torch, or be a variant of
Selene "moon", or a compound related to
Helios "sun". There's no consensus however and it may not be Hellenic at all, which opens up a wide range of poorly recorded languages neighbouring or preceding early Greek in the Aegean.
On the other hand Illyana may not be a variant of
Helene at all, depending on the source. If Russian it may be a feminine diminutive of
Ilya, which is the Russian development of Hebrew (E)L(e)Y(a)H, in English
Elijah, meaning "(my) god (lit. power) is Yah (an abbreviation of name of the God of
Moses)".