Kucsulain: What nationality is this last name?

Answer 1

I guess it's from Finland (Suomi) and that in origin it was the longer name Kucsulainen, then shortned as Kucsulain.
Ending -nen (or -lainen/läinen) has originally meant a person who lives in a certain kind of place. For example:

Mäki = hill; Mäkinen = someone who lives on a hill.
Pelto = field; Peltonen = someone who lives by a field.

The first part before -nen can also be the name of a farm or a village or a province or even a country. For example:

Karjala = Carelia, a province in eastern Finland; Karjalainen = one who lives in Carelia.
Suomalainen (Finnish), Ruotsalainen (Swedish), Venäläinen (Russian) and Virolainen (Estonian) are very common surnames in Finland.

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Answer 2

At a guess, Turkish.