Your version of Moodle is?
The block plugin
https://moodle.org/plugins/view.php?plugin=block_formal_langs
is compatiable up to version 2.8, 2.9 as shown on page linked below:
https://moodle.org/plugins/pluginversions.php?plugin=block_formal_langs
Do you have the right version of plugin for your version of Moodle?
It is also a part of sets:
Perl-compatible Regular Expression
https://moodle.org/plugins/browse.php?list=set&id=28
and
Correct Writing Question Type:
https://moodle.org/plugins/browse.php?list=set&id=30
It's my understanding of sets to mean more than one block/mod needs to be
uploaded and installed for something to function.
Your Fatal error gives a clue.
So this to suggest you've not gotten all of a 'set' to have the block work?
'spirit of sharing', Ken