Hi Didier,
As of now this is not possible through the plugin. However you can use CSS to achieve this.
Go to WP-Admin > Settings > Custom Error Pages, switch to the “Text” tab and enter error messages in all languages and wrap them in classes denoting their languages.
Example:
<div class="english-error">You don't have permission to access this resource</div>
<div class="french-error">Vous n'avez pas la permission d'accéder à cette ressource</div>
<div class="german-error">Sie haben keine Berechtigung, auf diese Ressource zuzugreifen</div>
Then open the custom CSS editor for your theme and enter the following CSS:
html:lang(en) .french-error,
html:lang(en) .german-error {
display: none;
}
html:lang(fr) .english-error,
html:lang(fr) .german-error {
display: none;
}
html:lang(de) .english-error,
html:lang(de) .french-error {
display: none;
}
You can use this solution for now while I try to make this plugin compatible with PolyLang.