One possibility is to avoid having dead keys and type accented letters with a Compose key instead. For example, type Compose " a to enter ä. You'll have to choose a keyboard layout option that includes a Compose key; a common choice is the key to the left of the right Ctrl key (which I think Windows calls Menu). The advantage of Compose is that you get to be able to type many fancy characters without changing your main layout. The disadvantage is that it takes three presses instead of two for an accented letter.

Alternatively, you can configure the effect of dead keys by creating a file called .XCompose (note CApitalization) in your home directory and listing the combinations you want. Something like:

include "/usr/share/X11/locale/en_US.UTF-8/Compose"

<dead_acute> <space> : "'" apostrophe
<dead_acute> <exclam> : "'!"
<dead_acute> <quotedbl> : "'\""
# etc...
<dead_acute> <a> : "á"
<dead_acute> <b> : "'b"
# etc...

The syntax is fairly simple: the sequence of keys to the left of the : is turned into the string between "" on the right of the :. The extra word on the right is the keysym (i.e., the name of the key) corresponding to this sequence of keys; it's not terribly important. Anything from a # to the end of the line is ignored.

Change the first line to point to the system file if your distribution puts it in a different place. You can look in this file for more syntax examples. <Multi_key> is another name for the Compose key.

On the left, the names between <angle brackets> are keysyms. You can find the list of keysyms in /usr/include/X11/keysymdef.h (this file is in a development package, e.g., x11proto-core-dev on Ubuntu).

Yes, that's a lot of typing, but you should be able to automate much of it by copy-pasting chunks of keysymdef.h and doing a few clever mass replacements. Something that will help is that you can reuse the hexadecimal code on the right to make the right-hand string thanks to the "\xdd" syntax: turn e.g.

#define XK_exclam                        0x0021  /* U+0021 EXCLAMATION MARK */

into

<dead_acute> <exclam> : "'\x21"

Finally, you can set up a keyboard layout with dead keys and a Compose key. Being German, you might set up only " as a dead key. If you're going that route, the simplest option is to use the GUI to set up a keyboard with no dead key and an .Xmodmap file for the dead key, containing something like

keysym apostrophe = apostrophe dead_quotedbl

Possible solution in MS Office

Change Normal style and apply the desired language setting.

Select English (U.K.).

(and is set to Polish when opening new files, contrary to what the popup says).

It solves the above issue. Every time you launch Word, it should be set to English (U.K.).

Even if at this point I choose English UK and click Set as default the proofing language still changes to Polish as soon as anything is typed

When you type in Polish character, it may recognized as Polish but spelling and grammar are not checked. Only English (U.K.) one would be checked.

Related Answer: https://superuser.com/a/362894/163596