During installation some user-defined dictionaries are installed. In a parallel installation with all installed dictionaries these user-defined dictionaries are affected: en-GB [English (UK), en-US [English (USA)], sl [Slovenian], technical [All]. Generally I expect that a user-defined dictionary can be deleted and modified. Otherwise it is not user-defined. This does not work with the mentioned dictionaries.
Steps to reproduce:
 Display the user-defined dictionaries with: Tools > Options... > Language Settings > Writing Aids.
 Select one of the mentioned dictionaries. It can't be deleted by clicking “Delete”. The button is greyed. Expected: Deletion of user-defined dictionary possible.
 Select one of the mentioned dictionaries and Click “Edit...”. An edit dialogue opens.
 Select a word and try to delete it. This is not possible. The button “Delete” is greyed. Expected: Deletion possible.
 Type in a new word into the field “Word”.
 Try to insert this word into the dictionary. This is not possible. The button “New” is greyed. Expected: Insertion possible.
I wonder if it makes sense to install these dictionaries as user-defined dictionaries. I think it's better to install them as part of the general language dictionaries, so that they don't appear in the options dialogue.
Some words according to the “IgnoreAllList” dictionary: It took me some time to understand this 'dictionary'. Its main characteristic is that it is temporarily, it is always deleted when you close LibreOffice. So it's not really a dictionary. In order to emphasize this characteristic I propose to change the name of this 'dictionary' to “Temporary Word List” or something similar. Furthermore it should be possible to localize this name.
Operating System: Windows 7
Version: 126.96.36.199 release
The main target of this report is the problem with dictionaries which can't be modified or deleted. Hence summary renamed again.
I also see this issue in LibreOffice 188.8.131.52.
The pre-installed "user-defined" dictionaries (or wordbooks) can be found in the
following directory: <LO_install_dir>/share/wordbook/ as *.dic files.
On Windows I get four pre-installed dictionaries: en-GB.dic, en-US.dic, sl.dic, technical.dic, with dictionaries for all languages selected in the installation wizard during installation.
On Linux (e.g. Debian and Linux Mint) I see only technical.dic although language support for both en-US and en-GB has been installed.
The pre-installed wordbooks can't be modified or deleted by standard users as they are not allowed to change or delete installed program files.
I'd suggest that the pre-installed dictionaries have some additional mark in the User-defined dictionaries list so that they are distinguished from real user-defined dictionaries.
I also see this issue in LibreOffice 184.108.40.206.
BTW, the summary used double negative which in standard English produce an affirmative (e.g. see https://en.wikipedia.org/wiki/Double_negative and http://dictionary.cambridge.org/dictionary/english/neither).
I still see this issue in LO 5.3.0Beta1 - tested on MS Windows 10 Pro.