I have the Ubuntu 20.04 LTS, and for me the same line was actually uncommented there.  But your finding is a good answer to the future seeking people. Thank you for updating the solution to help the community.

I had to uncomment this line of the envvars file located at etc/apache2/ envvars (line 28):

## Uncomment the following line to use the system default locale instead:
. /etc/default/locale

Restart Apache and TADA

Also Setup->System Diagnostics
Any inconsistency?

Althoug, which folders are you thinking about regarding the permission issues?

Generating locales (this might take a while)...
  de_AT.UTF-8... done
  de_BE.UTF-8... done
  de_CH.UTF-8... done
  de_DE.ISO-8859-1... done
  de_DE.UTF-8... done
  de_DE.ISO-8859-15@euro... done
(...)

reboot & still everything in gernan
Running out of ideas.

and choose the locales you need generating.

And the iso 8859 1 didn't support it.

sudo locale-gen de_DE.ISO-8859-1

Run this command on our Ubuntu terminal

Here some answers:

1) 201,8 kB
Content of *.po :  eg.
<code>
msgid "Description"
msgstr "Beschreibung"
</code>
-> Seems to be ok to me !

2) Very good idea. I have indeed the following:

[21-May-2021 22:25:09 Europe/Berlin] 0:gabriel:session.inc:315: Function get_magic_quotes_gpc() is deprecated
-> don't think that this is related

[03-May-2021 20:38:00 Europe/Berlin] 1 language code "de_DE", encoding "iso-8859-1" not supported by your system

-> I think that this is related...  but I have no idea, how this could be solved.
I understand the message like... there is a problem on the Ubuntu side.

I meant the version 2.4.1-3 is the same as the version in the file name de_DE-2.4.1-3.mo which it is. Your installed_languages.inc seems fine to me. Two things you might want to do:

1) What is the size of the de_DE-2.4.1-3.mo file? Do you see German Language when you open de_DE-2.4.1-3.po file with a text editor?
2) Do you have any error in ..../tmp/errors.log?
3) Any of the other languages (.ie Tamil) is working?

Lastly try  to remove and reinstall the language (from FA) again ..if not done already.

Lets see what you have..

(without the .mo extension.)

Hi,
I'm not sure what you mean with that remark.
In the folder there are 2 files:
$dir
FrontAccounting_files/lang/de_DE/LC_MESSAGES$ dir
de_DE-2.4.1-3.mo  de_DE-2.4.1-3.po

But both have extensions, as you see above.

Several reboot didn't change anything.

Do you have any other idea?

This is the content of the installed_languages.inc:

$installed_languages = array (
  0 => 
  array (
    'code' => 'C',
    'name' => 'English',
    'encoding' => 'iso-8859-1',
  ),
  1 => 
  array (
    'name' => 'Swiss German',
    'package' => 'de_CH',
    'code' => 'de_CH',
    'encoding' => 'iso-8859-1',
    'version' => '2.4.1-3',
    'path' => 'lang/de_CH',
  ),
  2 => 
  array (
    'name' => 'German',
    'package' => 'de_DE',
    'code' => 'de_DE',
    'encoding' => 'iso-8859-1',
    'version' => '2.4.1-3',
    'path' => 'lang/de_DE',
  ),
  3 => 
  array (
    'name' => 'Tamil',
    'package' => 'ta_IN',
    'code' => 'ta_IN',
    'encoding' => 'utf-8',
    'version' => '2.4.2-3',
    'path' => 'lang/ta_IN',
  ),
);

$dflt_lang = 'de_DE';

Thx.
Did that, without any effect.

See the result:

locale -a

de_DE
de_DE.iso88591
de_DE.utf8
de_IT.utf8
de_LI.utf8
de_LU.utf8
deutsch
(...)
among others

No effect on the frontend...

Did you restart your apache or machine to take effect.  Actually it will work. But sometimes not immediately. Try to check other languages like Arabic  and even Tamil too.  And than  set German again .  It might refresh to take effect.