For any settings in the <appSettings section of Web.config, you should never modify them or add them in Web.config, you should put custom settings in user.config where you will not lose your custom changes. Web.config has default settings, and you put your overrides in user.config.
Web.config already had a setting <add key="UseCultureOverride" value="false" />, so you should not add a new copy of it, the one that lives lower in the file wins in this case, so if you added it above the existing setting it would have no effect.
We do not add overrides for you in user.config except for a few special cases where default have changed and for backward compatibility we left the old defaults in Web.config but added overrides in user.config so new sites will use the preferred new settings by default.
For more info see Web.config Guide.
Hope it helps,