650 surname Sedlak listed in SSDI.
This means perhaps 1,000 surname bearers living in the U.S.
And I can think of a number of alternative spellings because
immigrants often changed surnames upon arrival in U.S.

However, Sedlak meaning farmer, is the 37th most popular surname
in the Czech Republic. Literally, thousands bear this surname in Europe.

Karoly also is the Hungarian given name for Charles (E)
Karol (S)

There was no country called Czechoslovakia until 1920.

So surnames born were born either in :
Czech-Bohemia was an Austrian Crownland and also part of the
Austro-Hungarian Monarchy(1867-1918). Here German and Czech names
were used because this region was under Austrian control.

Before WWI, Slovakia was part of Upper-Hungary (Felvidék) and
Hungarian names were used for towns and counties.
Part of Austro-Hungarian Monarchy (1867-1918).

In 1920, a newly-formed country of Czechoslovakia was created from the
Austrian Crownlands (Bohemia, Moravia and Austrian-Silesia) and a
portion of Upper-Hungary (Slovakia and Karpatho-Ukraine).

If you can locate surname on list you can Write Letter.
Copy of SS application form SS-5 usually lists town of origin, parent's
names, and mother's maiden name.
You will have to locate town of origin inorder to differentiate your surname records in Europe.

Or, if surname was naturalized, check Declaration of Intentions which
usually lists town of origin.