Starting by looking at admin/catalog/attribute.php it wouldn't be many strings left in it after cleaning up.

// Text

$_['text_success']           = 'The changes was successfully saved.';

$_['text_list']              = 'List';

$_['text_add']               = 'Add';

$_['text_edit']              = 'Edit';

// Column

$_['column_name']            = 'Name';

$_['column_attribute_group'] = 'Group';

$_['column_sort_order']      = 'Sort Order';

$_['column_action']          = 'Action';

column_attribute_group should be changed into column_group ?

// Entry

$_['entry_name']            = 'Name';

$_['entry_sort_order']      = 'Sort Order';

Note! Is there any need for both column_name and entry_name, in English and other languages? In Norwegian it's not.
At least there is no need for both column_sort_order and entry_sort_order...

// Error

$_['error_permission']      = 'Warning: You do not have permission to perform the selected action!';

$_['error_name']            = 'Name must be between 3 and 64 characters!';

$_['error_product']         = 'Warning: The selected item cannot be deleted as it is currently assigned to %s products!'

Btw! I think we could skip the "Warning:" too. The text is already red isn't it? Anyhow it's not concise in the error messages now.

For example, I am happy to have separate variables like

column_name and entry_name

We should keep them, because for example at the product form > Options I am using column_xx with ... for addtional explainations.
Further it separates the logic between entries and display variables.

What dou you think?

Repeated variables.
For example:

text_success

is used overall 103 times, all with the same meaning:

Record successfully edited

There is only one exception: extension/installer.php is using an additional explaination (here the German text):

$_['text_success'] = 'Erweiterung wurde erfolgreich hochgeladen, um sie anzuwenden müssen jetzt die Anpassungen angewendet werden.
Dazu bitte a href="/forum?route=extension/modification&token='%20.%20$_SESSION['token']%20.%20'" hier a klicken und auf Button Aktualisieren klicken';

(code is stripped from the system here, that is why it looks strange)

Warning, Attention, Note

There are all used at several places - and at least useless.
Yes, I vote for deleting them.

Michael wrote:We should keep them, because for example at the product form > Options I am using column_xx with ... for addtional explainations.
Further it separates the logic between entries and display variables.

I don't follow on this one, could you illustrate it with some English examples?

Michael wrote:There is only one exception: extension/installer.php is using an additional explaination (here the German text):

Could/Should be changed into $_['text_installer_success'] then.

Changing it into text_installer_success requires a change in the controller.
Leaving it as it is - but only in that file - will override the default vars.

Where we are at my next question: using

$data = $this->language->all();

will load all language vars at once instead of calling several times (e.g.)

$data['text_database_header'] = $this->language->get('text_database_header');

This would reduce the controller lines by approx. 20%