Moodle uses sentence case so it should probably be:
$string['filtername'] = 'FontAwesome icon filter.';
Also - the plugin name doesn't usually include the plugin "type" - in your case "filter"
so it might be better to be:
$string['filtername'] = 'FontAwesome';
or maybe even something like:
$string['filtername'] = 'FontAwesome icons.';
Probably been there since Julian created it and no-one had noticed :-)
Couple of things wrong with the filtername:
$string['filtername'] = 'FontAwesome Icon Filter.';
Moodle uses sentence case so it should probably be: $string['filtername'] = 'FontAwesome icon filter.';
Also - the plugin name doesn't usually include the plugin "type" - in your case "filter" so it might be better to be: $string['filtername'] = 'FontAwesome';
or maybe even something like: $string['filtername'] = 'FontAwesome icons.';
Probably been there since Julian created it and no-one had noticed :-)