Does this problem persist using 7.1.2?
https://www.libreoffice.org/download/download/

What about latest nightly?
https://dev-builds.libreoffice.org/daily/master/current.html

Same problem with 7.1.2 an Dev Build. For Dev Build there is only the language pack 7.1.2 available, which obviously wont't work...

"LO main application 7.0.5 is correctly installed in the applications folder near the old version (keeping both until I see that the new version works) and starts fine – thougt in Englisch."

Can you send a screenshot of what that looks like in Finder please. Can you try and remove all existing LO installations, then install 7.0.5 or 7.1.2 (without altering anything) and run the matching language pack installer.

Does that then work?

Created attachment 171118 [details]
screenshot Finder

Created attachment 171119 [details]
screenshot old version

Created attachment 171120 [details]
screenshot new version

Thank you Steve for your assistance
I have manually renamed the old version to distinguish it from the newly installed one. Interresting: the detail info of the old version shows the correct version number (7.0.4.2), while the new version shows just ---, see attachments.

I am a bit reluctant to trash all my LO installations for test purpose, because I have several prefs, I dont't want to loose, e.g. a DB-connection to create invoices from my main client and job spreadsheet

Sure, maybe just retry to re-download 7.1.2 + language pack for that download and see is that installs. Also curious then to hear, does this test fail on both 10.13 and 10.14 for you?

I have downloaded 7.1.2.2 and the corresponding LP on the Mac Mini with OS 10.13.6 which has already LO 6.4.7002 installed. The 7.1.2 installer asks me correctly, if I want to replace the existing version or keep both. I click keep both an the program installs correctly in the Applications folder where it appears as "LibreOffice 2" (while the 6.4.7 version is labeled "LibreOffice"). 

when I try to install the de-LP, the installer doesn't automatically find the newly installed app an tells me to choose it manually in a next step. In the dialog I chose "LibreOffice 2" and get the error message, that this is not a valuable 7.1 installation of LO.

As I wrote, this problem appears on two machines, one with OS 10.14 and one with 10.13., no matter it I want to install the LPs for LO 7.0.5 or 7.1.2.2

The only divergence from a out-of-the-box Mac OS installation is, that I don't use the Spotlight search machine and therefore prevent the indexing of my HD (by telling Spotlight not indexing it). But I can't imagine, that this has anything to do with the problem.

Try replacing the 6.4 app instead. Does that then result in the same problem? After the test you can always revert to 6.4 if you need that version.

Did a clean install on the MacBook Air with OS 10.13. Deleted ALL LO files with App Cleaner – really nothing left. Installed 7.1.2.2 and LP – installation of App ok, installation of LP fails.

I deleted again everything and went back to 6.4. LP can be installed without a problem.

Correction: The MacBook Air is on Mac OS 10.14

Please open Terminal app and send the output of the following command:

    mdls /path/to/LibreOffice.app


You can enter mdls and then drag the LibreOffice.app from your programs folder into terminal to get the path.

/path/to/LibreOffice.app: could not find /path/to/LibreOffice.app.

On MacMini with Mac OS 10.13. and LO 6.4.7002 installed (and working)

Please ignore 6.4 for now. This is a problem with 7.1.2 and it should work.

"/path/to/LibreOffice.app: could not find /path/to/LibreOffice.app" what do you mean by that? I explained how this command is handled.

Also the command should be executed using LO 7.1.2 not 6.4.x.

It would be interesting if you could temporarily enable spotlight indexing and see if that changes anything. It may seem odd but those details do make a difference and can lead to all sorts of behavior.

1) Clean install of LO 7.1.2.2
2) open terminal app, typing mdls
3) pulling LO-app to terminal window
4) pushing return key
5) -bash: mdls/Applications/LibreOffice.app: No such file or directory

Will now try the same with reenabled spotlight function

You are missing a space between "mdls" and "/path/to/LibreOffice.app"
see: https://bugs.documentfoundation.org//show_bug.cgi?id=141622#c16

Sorry for that one. Now I get:
You have mail.
Emanuel-Scheideggers-Mac-mini:~ emanuelscheidegger$ mdls /Applications/LibreOffice.app 
_kMDItemDisplayNameWithExtensions  = "LibreOffice.app"
kMDItemAlternateNames              = (
    "LibreOffice.app"
)
kMDItemAppStoreCategory            = "Produktivität"
kMDItemAppStoreCategoryType        = "public.app-category.productivity"
kMDItemCFBundleIdentifier          = "org.libreoffice.script"
kMDItemContentCreationDate         = 2021-03-24 20:45:40 +0000
kMDItemContentCreationDate_Ranking = 2021-03-24 00:00:00 +0000
kMDItemContentModificationDate     = 2021-03-24 20:45:40 +0000
kMDItemContentType                 = "com.apple.application-bundle"
kMDItemContentTypeTree             = (
    "com.apple.application-bundle",
    "com.apple.localizable-name-bundle",
    "com.apple.application-bundle",
    "public.directory",
    "public.executable",
    "com.apple.application",
    "public.item",
    "com.apple.package",
    "com.apple.bundle"
)
kMDItemDateAdded                   = 2021-04-15 11:31:12 +0000
kMDItemDateAdded_Ranking           = 2021-04-15 00:00:00 +0000
kMDItemDisplayName                 = "LibreOffice"
kMDItemExecutableArchitectures     = (
    "x86_64"
)
kMDItemFSContentChangeDate         = 2021-03-24 20:45:40 +0000
kMDItemFSCreationDate              = 2021-03-24 20:45:40 +0000
kMDItemFSCreatorCode               = ""
kMDItemFSFinderFlags               = 0
kMDItemFSHasCustomIcon             = 0
kMDItemFSInvisible                 = 0
kMDItemFSIsExtensionHidden         = 0
kMDItemFSIsStationery              = 0
kMDItemFSLabel                     = 0
kMDItemFSName                      = "LibreOffice.app"
kMDItemFSNodeCount                 = 1
kMDItemFSOwnerGroupID              = 80
kMDItemFSOwnerUserID               = 501
kMDItemFSSize                      = 1
kMDItemFSTypeCode                  = ""
kMDItemInterestingDate_Ranking     = 2021-03-24 00:00:00 +0000
kMDItemKind                        = "Programm"
kMDItemLanguages                   = (
    ca,
    cs,
    da,
    de,
    el,
    en,
    es,
    fi,
    fr,
    hr,
    hu,
    id,
    it,
    ja,
    ko,
    nl,
    pl,
    pt,
    ro,
    ru,
    sk,
    sv,
    th,
    tr,
    uk,
    vi
)
kMDItemVersion                     = "7.1.2.2"
Emanuel-Scheideggers-Mac-mini:~ emanuelscheidegger$

And language pack installation for that version download still fails right after running the mdls command?

Good news, Steve!
Re-enabling Spotlight and rebuilding the Spotlight-index did the trick. The LP for 7.1.2.2 installs now without a hichcup. It seems, that the LP-installer script  uses the Spotlight index to identify the LO main app.
Thanks again for your assistance.

Setting to worksforme as there was no commit for a fix. Glad this is working as expected for you.