I believe this is related, but not quite the same as long-standing bug #599860.

Short explanation: print("ФЫВА") from gjs console or first module (test.js for $ gjs test.js) works as expected, from any imported module it produces garbled output.

Simple test case. File 'test.js':

    print ("ФЫВА");
    imports.searchPath.push ('.');
    imports.test2.foo ();

File 'test2.js':

    function foo () { print ("ФЫВА"); }

Run as $ gjs test.js. Output:

    ФЫВА
    ФЫÐÐ

I believe this could be resolved in file 'gjs/importer.c' if after this code:

    if (!g_file_get_contents(full_path, &script, &script_len, NULL)) {
        return NULL;
    }

some decoding for 'script' buffer is added. As it stands, the buffer (raw bytes read from file) are passed to SpiderMonkey, which itself doesn't try any recoding.