Insert both characters if a dead key combination is not recognized (e.g. 'a → á, 'b → 'b)

I found a way to make this work in OpenSUSE, and I hope it won't be too different in Mint. Maybe your files will be in slightly different locations, but it shouldn't be too hard to find the right ones.

OpenSUSE uses ibus by default (or, at least, the Gnome version does), but I got rid of it and used xim. I don't know if Mint also uses ibus but, in any case, I'm posting the complete instructions. Skip/adapt the steps as necessary.

1. Uninstall ibus. In OpenSUSE, that's sudo zypper rm ibus. Logout and login again to kill the zombie daemons.

2. Install gtk3-immodule-xim. In OpenSUSE, it's sudo zypper in gtk3-immodule-xim.

3. Edit /etc/X11/xim.d/none. Set XMODIFIERS to @im=none. Set GTK_IM_MODULE and QT_IM_MODULE to xim.

4. cp /usr/share/X11/locale/en_US.UTF-8/Compose ~/.XCompose. If there's a .XCompose file in your home directory, X11 will use it. So now we can edit this file to customize the dead keys.

5. Now, the boring part. Edit the .XCompose file and create rules for each combination of dead keys, outputting the desired string. For example:

   <dead_acute> <b> : "'b"
   <dead_acute> <r> : "'r"
   ...
   

   Remember to create rules for uppercase letters, too (and punctuation, and anything you need).

6. If, like me, you also want to get cedilla (ç) instead of accented c (ć) when using <dead_acute> <c>, simply find and edit those lines too.

And there you have it. Logout and login again, and all the applications should handle input correctly!

EDIT: I'm aware that xim is old and buggy (it says so on the config file), but the above was the only way I could get everything working. Maybe uim also does the job.

On Ubuntu 14.04 I did the following:

1) Installed uim using the Software Manager, other packages like uim-xim, uim-gtk2, uim-gtk3 and uim-qt are auto installed. See https://launchpad.net/ubuntu/+source/uim.

2) Defined environmental variables by adding the next lines to ~/.profile, this way the custom compose key sequences only apply to the current user:

# Restart the X-server after making alterations using:
# $ sudo restart lightdm
# It seems only GTK_IM_MODULE or QT_IM_MODULE needs to be defined.
export GTK_IM_MODULE="uim"
export QT_IM_MODULE="uim"

3) To mimic Window US International keyboards I saved one of the following files at ~/.XCompose:

• https://gist.githubusercontent.com/guiambros/b773ee85746e06454596/raw/0ea6d7f7cf9a6ff38b4cafde24dd43852e46d5e3/.XCompose or
• http://pastebin.com/vJg6G0th

This worked for me after 1) restarting Ubuntu or 2) just the X-server by entering the following command in a terminal:

$ sudo restart lightdm

NB: Restarting only seems necessary after altering the ~/.profile file, alterations to ~/.XCompose will take effect the next time an application (Terminal, Gedit, etc.) starts.

To check whether the environmental variables are set right, enter the following command in your terminal:

$ printenv | grep IM_MODULE

---

Many thanks to:

• https://wrgms.com/using-xcompose-with-chrome-and-sublime-text

About custom compose key sequences:

• http://manpages.ubuntu.com/manpages/trusty/man5/XCompose.5.html
• https://help.ubuntu.com/community/ComposeKey

About custom keyboard mapping:

• https://help.ubuntu.com/community/Custom%20keyboard%20layout%20definitions

Having 'a and 'b behave differently sounds annoying. I recommend that you use a different way to input characters with diacritics. Instead of making ' and such dead keys, use a compose key. To enter 'a', you type ' a ' normally. To enter á, you type Compose ' a.

I'm not familiar with Mint's keyboard layout configuration interface but it should have an option to enable the Compose key somewhere. The usual choice is to use the “menu” key located to the left of the right Ctrl key (but that isn't always convenient on a laptop).

To keep things uniform, you can enable a Compose key on Windows with Wincompose.