As previously mentioned, the module which provided the "Language Options" panel has been removed as it was unmaintained. This was never provided by upstream KDE.

In regards to adding a Vietnamese keyboard layout, you should be attempting to do that through System Settings > Input Devices > Keyboard > Layouts.
In terms of setting the system to use Vietnamese as the primary language, you need to find "Vietnamese" under the Available languages list and add it to the preferred list - then move it above English and apply.

Thank you for your patience, but I thought you said that it was removed by the OSes maker team, not KDE, didn't you?

I know that way. The problem is we don't use that input method. In Gnome, after installing IBus and the language variant IBus, go to Text Entry and you can find the other variant input method, i.e. below Vietnamese is Vietnames (unikey).

Yes, they removed it (after adding it in themselves) because it was unmaintained (by it's upstream author).

I'm afraid i'm totally unfamiliar with the mechanism you're describing - the only layout configuration KDE offers is that module. If you need to configure something IBus specific, you will need to use the appropriate IBus configuration interface which it includes (assuming it does so) to make that change.

Just for making sure. So the problem is that IBus' authors stop making this project, and because of that the authors of KDE OSes (at least three ones I have used: Kubuntu, Mint and Netrunner) stop including it into the OSes? I have a bit confuse here. Doesn't that mean KDE does not support it? I see that Gnome can go along with it very well. Do other users in other country ask about IBus when they using KDE like me?

This post seems to be off topic. Should I create another thread?

The project in question was written by a 3rd party - not the IBus or KDE developers. You are correct however that it has stopped being developed - and has therefore been removed from Kubuntu upstream (which is why it disappeared from Mint and Netrunner as they are Kubuntu derivates).

KDE supports IBus to the extent Qt does so - it is usable for entering input, however no options for configuring it are provided. You need to use the native IBus methods to do that. GNOME likely "supports" it because IBus includes a Gtk control module - i'd suggest trying that. I see no reason to create a new topic though.

So in order to use IBus like in GNOME, I have to install Gtk control module, right? Does KDE support that? Do you think that if I install Ubuntu, then install KDE and remove Unity (or GNOME if I install the GNOME derivation), the problem will be solve?

You don't need to remove KDE, install Unity or GNOME. All you need to do is install the appropriate IBus Gtk control module - it should be completely usable within KDE.

Google around with keyword "IBus Gtk control module" give me no clearly answer. The only informatic answer I got is this one: http://code.google.com/p/ibus/wiki/FAQ, but when I use sudo apt-get install gnome-icon-theme as it suggests, it see that I have already installed the newest version.

It should simply be a case of launching it then. I'm not sure what the command to do this would be however.
Otherwise you might want to try Kimpanel instead, see https://wiki.archlinux.org/index.php/IBus#Kimpanel for more information (although i'm not sure what it's maintenance status is).

That one is Keyboard Input Method. You had helped me in this thread: https://forum.kde.org/viewtopic.php?f=22&t=120506&p=308353#p308353

I can't install Kimpanel, it says unable to locate it. Do I need to add a respository or source?

Kimpanel is part of kdeplasma-addons I believe, so it should be included if you install that.

OK, back to the question:

ooker wrote:So in order to use IBus like in GNOME, I have to install Gtk control module, right?

Your answer is no, because they already have. But it doesn't work like in Gnome.

Let's continue this issue in the other thread - as they're both working to solve the same issue (I think, please correct me if i'm wrong).

Well, I think the GUI is no longer a problem anymore. The problem is deeper than just interaction. Thanks for patiently answering me.