German and Spanish special characters [closed]

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The LibreOffice version is 5.4.1.2

The operative system is Kubuntu 17.10

When I want to type in LibreOffice Writer the German characters "ä, ö, ü, ß", or the Spanisch characters "á, ó, ñ", the input is ignored, as if I had not typed anything at all and the next letter I type is displayed.

As you can see here, I can type these characters perfectly well inside this browser and all other software.

Can anyone give me a helping hand please?

Thanks!