WiX Burn bootstrapper cancel dialog localization

Question

I have successfuly created a localized installer and bootstrapper setup using WiX. There is just one problem with the bootstrapper localization. Everything is fine except the values of button captions in a dialog that appears when the installation is being canceled (exit confirmation yes/no). There are no string elements in the bootstrapper theme wxl files that would allow to change these values. In the MSI installer this was possible by setting "WixUIYes", "WixUINo" values.

I have tried to add the following elements to my localized theme file, but without success.

<String Id="WixUIYes" Overridable="yes">some_value1</String>
<String Id="WixUINo" Overridable="yes">some_value2</String>

Is there a way to change the caption values of mentioned buttons? Their values are currently in the system language.

This is my WXS file:

<?xml version='1.0' encoding='windows-1250'?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi"
     xmlns:bal="http://schemas.microsoft.com/wix/BalExtension">

    <Bundle Name="Test" Version="1.0.0.0" UpgradeCode="MY-GUID">
        <WixVariable Id="WixStdbaThemeXml" 
                     Value="Resources\HyperlinkTheme.xml" />
        <WixVariable Id="WixStdbaThemeWxl" 
                     Value="Resources\LocalizedHyperlinkTheme.wxl" />

        <BootstrapperApplicationRef Id="WixStandardBootstrapperApplication.HyperlinkLicense" >
            <bal:WixStandardBootstrapperApplication LicenseUrl="" SuppressOptionsUI="yes" />
        </BootstrapperApplicationRef>
        <Chain>
            <ExePackage Id="VCRedistExe"
                        Cache="no"
                        Compressed="no"
                        Vital="yes"
                        SourceFile="vcredist_x86.exe" />
            <MsiPackage Id="MyAppMsi"
                        Cache="no"
                        Compressed="yes"
                        Vital="yes"
                        SourceFile="MyApp.msi"
                        DisplayInternalUI="yes" />
        </Chain>
    </Bundle>
</Wix>