You can do this simply with some custom CSS. The flag already exists, and you can simply shift the sprite location over one.
Go to Settings -> Google Translate Plugin -> Add CSS Styles, and add the following CSS depending on the Flag size you’re using.
24px
#flags.size24 a[title="English"], .tool-container .tool-items a[title="English"] {
background: url("/wp-content/plugins/google-language-translator/images/flags.png") no-repeat scroll -117px -351px rgba(0, 0, 0, 0);
}
22px
#flags.size22 a[title="English"], .tool-container .tool-items a[title="English"] {
background: url("/wp-content/plugins/google-language-translator/images/flags22.png") no-repeat scroll -117px -351px rgba(0, 0, 0, 0);
}
20px
#flags.size20 a[title="English"], .tool-container .tool-items a[title="English"] {
background: url("/wp-content/plugins/google-language-translator/images/flags20.png") no-repeat scroll -118px -352px rgba(0, 0, 0, 0);
}
18px
#flags.size18 a[title="English"], .tool-container .tool-items a[title="English"] {
background: url("/wp-content/plugins/google-language-translator/images/flags18.png") no-repeat scroll -119px -352px rgba(0, 0, 0, 0);
}
16px
#flags.size16 a[title="English"], .tool-container .tool-items a[title="English"] {
background: url("/wp-content/plugins/google-language-translator/images/flags16.png") no-repeat scroll -120px -353px rgba(0, 0, 0, 0);
}
