Oh I can recognize multiple languages, sometimes even differentiate those which use the same (non-Latin) character set. And I seem to have a talent for listening to a native speaker and assuming hiser accent into my own speech. I may not know word one in the languages in question, but I can still pretty much differentiate them.
My suspicion is that "The Black Cat" (that is what the elements of AnCatDubh which I recognize translate into from the Gaelic) is one of those folks who collect languages the way some of us collect photographs.
'Takk for sist' can be used as a greeting if you've actually met someone before, but never when you first meet a new person. You're basically thanking them for the good time you had together the last time you met. You can also combine the two: "Hei, og takk for sist!".
'Heia' is used as a cheer, and also as an informal greeting.
"Hallo" comes off as slightly more formal in writing. In speech, there is little difference, and the choice depends on dialect and personal preference.
In particularly formal situations, "God morgen/dag/aften" would be my choice.
When a greeting is used questioningly (Hello? Is anybody there?), we tend to default to "hallo" just because it makes the tone change more clear than the one-syllable "hei".