Agreed with the fix, but there is a codestyle issue. You need to add a space before the closing bracket:
Change $db->quote('*') .')' to $db->quote('*') . ')'

Also please add the information to reproduce the issue and test the patch.

Seems there are still something wrong with the PR (based on code review):

1. I think we don't need to have $query->where('a.created_by = ' . (int) $result->user_id) because It is added before already https://github.com/joomla/joomla-cms/blob/staging/components/com_contact/models/contact.php#L381 and I don't see any reasons that only be added if the site is multilingual.

2. Seems we are using IN clause for language query in other places in core, not using OR. A quick google search also show that IN has better performance in OR. So maybe we should change OR to IN

So the code in the PR could be changed to:

if (JLanguageMultilang::isEnabled())
{
    $query->where('a.language IN (' . $db->quote(JFactory::getLanguage()->getTag()) . ',' . $db->quote('*') . ')');
}

Made a PR at nn-medienagentur#1 . In case we agree with this change @nn-medienagentur can merge the PR to save time.

Moving to Information required until @nn-medienagentur have a look into here. Thanks

@joomdonation you're right. I have tested it, and it work great with IN and remove created_by in if clause, thanks.

Removed Information Required tag

So.... How to test it :-)

@test
This works fine here.

To test:
Display a contact for USER1 in frontend. "Show User Articles" should be set to "Show" in the menu item. (Don't forget that, in multilanguage, a user/author should have only one contact to which is assigned language 'All' OR one contact for each published Content Language.)
Create an article set to All Languages and assign it to another user (USER2) than the one displayed by the menu item.

That article will be displayed by the menu item in the list of Articles for USER1.
Patch and test again: it will not any more as it does not belong to that user.

@infograf768 i tested like you described the issue but can´t reproduce it.
Steps:

• Created two Users (User 1, User 2) Both have language set to "All"
• Created an Article with Language Set to "all" for User 2
• Created two Articles for User 1 (en-GB, de-DE)
• Created a Menüitem for Displaying Contact for User 1 (Show Articles is marked) -> The Show Articles Slide only shows the articles of user 1 After deleting the two Articles from User 1 the slide is not visible anymore.

~ Any other instructions, did i miss something ?

Cannot reproduce this error.
Not a multilingual site, nor on a single language site.

used version J!3.5.1
followed the test instructions from @infograf768

Several people have stated that they cannot reproduce the issue. This could be because it no longer exists and has been solved elsewhere or it is down to the test intructions not being detailed enough.

Please can you check if this is still an issue and update with detailed test instructions or this will be closed in a few weeks.

Thanks

Here is the description to reproduce the issue.

1. create a User1
2. create a User2
3. create a article like "User1 Language ALL" and select created_by the User1 Language ALL
4. create a article like "User2 Language UK" and select created_by the User2 Language en-GB
5. create a Contact "User2" and liked the Contact with User2 Language ALL
6. Add a new MenuItem from type Contacts - > Single contact add select the User2 and change in "Contact Display Options" -> Show User Article to "show" language for the Menu can be ALL
7. IMPORTANT now you have do enable the Plugin "System - Language Filter"

Now you see on the Frontend in the Single Contact View the Article from User 1

I have tested this item successfully on 47cc301

Tested again with new instructions.
Now i could reproduce the error and test patch.
TEST OK

Rtc

This PR has received new commits.

CC: @infograf768, @mikeveeckmans

@nn-medienagentur what are the changes?

@mikeveeckmans nothing, why are you asking?

Just one comment before I merge this, I check what is faster "or" vs. "in" and it seems that "in" is faster.