Localizing stored functions by replacing placeholders in their body

Hello,

I have an app using PostgreSQL 13.2, in 6 different human languages (each
using different database, but same source code).

Currently to localize strings return/set by the stored functions I either
get localized strings from a table or maintain stored function source code
in 6 different languages.

This is not very comfortable and I would like to switch to using same
source code (regardless of the human language) for all stored functions.
And after deploying a database, just run few commands to replace
placeholders in the stored functions.

So I am trying:

CREATE OR REPLACE FUNCTION localize_hello()
RETURNS text AS
$func$
SELECT '$(hello)';
$func$ LANGUAGE sql IMMUTABLE;

And then:

update pg_proc set prosrc = regexp_replace(prosrc, '$\(\w+\)','Hi
english','g') where proname='localize_hello';

But the error is:

ERROR: permission denied for table pg_proc

So I connect as user "postgres" and then the command seemingly succeeds,
but when I call it, the delivered string is still old:

select * from localize_hello();
localize_hello
----------------
$(hello)
(1 row)

Is this a right approach? Do you please have any advice here?

Thanks
Alex