User's default language changes to the one that was selected on the logon screen. If "User's default" language is selected during login, the user`s default language will be changed to a language that is defined under Top > Configuration Director > Miscellaneous Settings > Regional Settings > New Person Default Settings
Such behavior was classified as the software issue PBAS-30138. It was fixed in OBAS 4.5.6.
To resolve the issue, please upgrade to 4.5.6 as described in How to upgrade OBAS system to the latest available version.