If there is not a standard character set for your language then you can invent one. The new charset will only be used by Aspell internally. If the option data-encoding is set to `utf-8', and your current locale character type is always set to `utf-8', then you can use UTF-8 for everything and not worry yourself that an 8-bit character set is being used internally. If your language has no more than 210 distinct symbols, including different capitalizations and accents, then Aspell can support it.
The first thing to do is to download the Aspell lang package (see Creating An Official Dictionary Package) and check if one of the provided charsets in this package will suite your needs. Non-standard character sets are provided for many scripts and languages. If not, then see the included README file for instructions on creating a new one. Version 0.1, and 0.2 of mkchardata will not work as the format of the character data file has changed.