are you sure you have to use the wordpress plugin translation stuff?
you somewhere include this i think:
<script>(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/en_EN/sdk.js#xfbml=1&version=v2.3";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>
if you just switch the “en_EN”-part of the javascript source to “de_DE” the buttons would be translated directly (or whatever languages i.e. “nl_NL” for netherlands -> https://en.wikipedia.org/wiki/List_of_Internet_top-level_domains#Country_code_top-level_domains)
edit: its in the pluginroot/js/simple-facebook-page-root.js file in line 12 the “en_US”. i’m not a programmer but maybe something like
var Language = "en_US";
if (LanguageVariableThatCanBeDefinedByTheUser!="")
{Language = LanguageVariableThatCanBeDefinedByTheUser}