Created attachment 78725 [details]
A file with Farsi characters.

Sorry that I could not find the appropriate product and component for this bug, thus, one should change them to correct values. Definitely it is not just for kate. I happens in all of kde applications.

What font are you using?
What happens if you increase/decrease the font size?
Can you check different fonts?

Created attachment 78731 [details]
The open source farsi fonts

Thank you for your quick reply. I am using the open source farsi fonts attached.
If I increase the font size or decrease it, it just scales the text up and down but does not affect the problem. It seems that there is an imaginary line just below the text which cuts it and prevents the character from being shown properly.
I have tested all the fonts and the situation is more or less the same. Although with somehow the problem is more noticeable and with some less.
I have set to use the fonts through /etc/fonts/local.conf:
<?xml version="1.0"?>
w<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- /etc/fonts.conf file to configure system font access -->
<fontconfig>
        <!--  Enable sub-pixel rendering
        <match target="font">
                <test qual="all" name="rgba">
                        <const>unknown</const>
                </test>
                <edit name="rgba" mode="assign"><const>rgb</const></edit>
        </match>
         -->

        <alias>
                <family>serif</family>
                <prefer>
                        <family>Bitstream Vera Serif</family>
                        <family>Homa</family>
                </prefer>
        </alias>
        <alias>
                <family>sans-serif</family>
                <prefer>
                        <family>Bitstream Vera Sans</family>
                        <family>Homa</family>
                </prefer>
        </alias>
        <alias>
                <family>monospace</family>
                <prefer>
                        <family>Bitstream Vera Sans Mono</family>
                        <family>Homa</family>
                </prefer>
        </alias>
</fontconfig>
For example, with the font Homa, the situation is much worse.

If I don't use the local.conf file, by default it uses a font which has the least problems but is super ugly.
In the file local.conf above, there is an error in the second line : "w" should not be there.

Also all taskbar entries with Persian names are not shown properly. Somehow they are shown like the way in a text file. The tag of a taskbar entry needs to be shown in the center, but It is too much near the bottom limit.

Created attachment 83923 [details]
A screenshot showing the problem

Hello,
I have attached a screenshot showing the problem.
As soon as I change the layouts, not only the Persian/Arabic characters are shown lower than needed, but also they change the way the English characters are shown. 
The problem is clear in Konsole and in the taskbar (The place the windows are minimized).

Not sure how to fix that in KDE 4.x, but for KF5 branch, it just works.

I can fully read the

سلام به همه آدمهای خوب دنیا

line

(In reply to Christoph Cullmann from comment #9)
> Not sure how to fix that in KDE 4.x, but for KF5 branch, it just works.
> 
> I can fully read the
> 
> سلام به همه آدمهای خوب دنیا
> 
> line

Thank you very much for the good news.

I will try to test the new framework and manage to install it. 
Just to check again, did you print the the entry in Konsole or put it in a mixed languge Kate document and the text was not cut?

Thank you again.

Hi,

The problem is not resolved in Kubuntu 16.04. Still part of Persian text is cut. That's in Kate and Terminal. I checked with a GTK editor and there was no problem, Persian text there displayed well.

Hamid

We just did improve further with that, look at bug 335079

Thanks. When is Kate updated then? Will it appear in system update?

It will appear with KDE frameworks 5.27.