The compiler has an option -Sc ... -Sc KSC

This -Sc option must be followed by one of the following encoding scheme identifiers: BIG5, DBC, EUC, GB, KSC, or SJC. When used, this option causes the compiler to assume that the input source is encoded using the specified encoding scheme. This allows the compiler to recognize double-byte characters used in string literals and comments.

The runtime has a configuration variable CODE_SYSTEM... although you may not need CODE_SYSTEM unless you plan to display double byte characters ... The runtime configuration variable CODE_SYSTEM tells the runtime if double-byte character data is being accepted or displayed, and which code system (that is, which standard for encoding Japanese and other Asian character sets, for example) is being used. Each code system has a range of values that it allows within each byte of a two-byte character, so identifying the code system allows the runtime to recognize character boundaries when it is processing double-byte data for ACCEPT and DISPLAY statements.

I do not know if this is true for Ecel, many programs like MSSQL / DB2 either use Locale or some environment setting to specify the types of characters that the table can have, so it may be the case where you cannot have English and Korean characters in the same table, or it may be that you need to change Locale in order to place Korean characters into the table.