How to have xRDP acknowledge current user's locale?

Question

My problem is that xRDP does not seem to recognize the ~/.pam_environment file of the user, who is being logged in.

What I did:

• I have set up a fresh Ubuntu Server 18.04.02 LTS
• I installed MATE as the desktop environment
• I installed xRDP
• I have created 2 more users

What I can do:

• I can successfully login from Windows (10) into a session for every user
• I can start mate locally just fine and login with every user into their own sessions

I installed German as an additional language in Menu > Control Center > Language Support and reordered German (Deutschland (Deutsch)) to be above English (fallback I assume).

Now when I log in with user1 locally, I see the MATE desktop environment in English, and when I login with user2, I see it in German.

However: when I log in via a remote RDP-session (via Xorg) from my Windows computer using the corresponding credentials, the MATE desktop environment stays English no matter what user logs in.

I suspect that xRDP is drawing its information about the locale to use from /etc/default/locale, which has the following content:

LANG="en_US.UTF-8"
LANGUAGE="en"
LC_NUMERIC="de_DE.UTF-8"
LC_TIME="de_DE.UTF-8"
LC_MONETARY="de_DE.UTF-8"
LC_PAPER="de_DE.UTF-8"
LC_IDENTIFICATION="de_DE.UTF-8"
LC_NAME="de_DE.UTF-8"
LC_ADDRESS="de_DE.UTF-8"
LC_TELEPHONE="de_DE.UTF-8"
LC_MEASUREMENT="de_DE.UTF-8"

According to this the language is expected to stay English.

Am I missing something?

Edit: adjustments in ~/.profile I thought were necessary, actually are not. It works regardless, because locally ~/.pam_environment is used. Via xRDP for whatever reason it seems not to be recognized.

Edit 2: Output from locale when user2 is logged in locally:

LANG="de_DE.UTF-8"
LANGUAGE="de_DE:en"
LC_NUMERIC="de_DE.UTF-8"
LC_TIME="de_DE.UTF-8"
LC_MONETARY="de_DE.UTF-8"
LC_PAPER="de_DE.UTF-8"
LC_IDENTIFICATION="de_DE.UTF-8"
LC_NAME="de_DE.UTF-8"
LC_ADDRESS="de_DE.UTF-8"
LC_TELEPHONE="de_DE.UTF-8"
LC_MEASUREMENT="de_DE.UTF-8"

Output from locale when user2 is logged in remotely:

LANG=en_US.UTF-8
LANGUAGE=en
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=de_DE.UTF-8
LC_TIME=de_DE.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=de_DE.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=de_DE.UTF-8
LC_NAME=de_DE.UTF-8
LC_ADDRESS=de_DE.UTF-8
LC_TELEPHONE=de_DE.UTF-8
LC_MEASUREMENT=de_DE.UTF-8
LC_IDENTIFICATION=de_DE.UTF-8
LC_ALL=