I have installed LibreOffice in German, but - using customized install - also added Dutch / Nederlands as additional language.
Throughout different parts of LibreOffice, some menu items or default document titles are now in Dutch / Nederlands instead in German.
Opening up Impress:
-> Dialogue for Templates shows "Openen" and "Annuleren" (Nederlands; WRONG here) and the title of the empty document is "Geen titel 1" (which is also Nederlands; WRONG here).
The menus are displayed completely in German (CORRECT), with exceptions regarding closing LibreOffice ("LibreOffice afsluiten" - Nederlands, WRONG) and saving files: "Datei" (File) menu and Burger menu both include the Nederlands "Opslaan" (WRONG) instead of the German "Speichern" entry. The file menu has a few more of those "opslaan" entries (WRONG), while the ribbon menu displays the correct German "Speichern" and "Speichern unter" (CORRECT).
Steps to Reproduce:
1. Install LibreOffice with custom install. Primary language German, additional language Dutch / Nederlands.
2. Start LibreOffice Impress (or Writer, similar behaviour).
see "Steps to Reproduce"
Localization should have been in German, only, unless explicitly selected otherwise.
User Profile Reset: Yes
As you can see below, the mixed localization also shows in the "About" dialogue:
Version: 220.127.116.11 (x64)
CPU-threads: 4; Besturingssysteem:Windows 6.19; UI-render: standaard;
Gebietsschema: de-DE (de_DE); Calc: group
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0
So German is your operating system language as well?
Yes, German is the Windows 10 OS setting, too.
However: A fresh install of the whole system (from scratch to Win 10 Pro Build 1709 = FCU) incl. new install of LibreOffice 18.104.22.168 and new user profiles does not show this bug, anymore. I cannot it reproduce it under these circumstances, anymore. Tested today. :-/
Before, it was a Creators Update release. LibreOffice had been updated multiple times (installing latest version on top of existing version using the installer) and at some point in time started to show that bug in that environment.
All right, then we can close with good conscience.