Note: plugin version 2.9.0 >

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To allow phpBB users to be transferred/migrated in WordPress using their non Latin language characters, it is required to add into the wp.config.php file the following line of code

define( 'WPW3ALL_USE_UNAME_NATIVE_LANG_CHARS', 1 );

after the line

/* Add any custom values between this line and the "stop editing" line. */

See more about or reply here: https://www.axew3.com/w3/forums/viewtopic.php?t=1920