Hi @NiklausBal, welcome to the Streamlit community!! 
Streamlit does not expose the option to change the placeholder text for st.multiselect. As such, there is no native option to change the placeholder because it is hard-coded in the frontend.
However, here is an unofficial CSS hack. I used a CSS selector to first make the default placeholder invisible. Next, I use another CSS selector to set the value of the placeholder. This CSS is injected into the app with st.markdown.
Solution
1 Insert the following snippet into your app:
import streamlit as st
change_text = """
<style>
div.st-cs.st-c5.st-bc.st-ct.st-cu {visibility: hidden;}
div.st-cs.st-c5.st-bc.st-ct.st-cu:before {content: "Wähle eine Option"; visibility: visible;}
</style>
"""
st.markdown(change_text, unsafe_allow_html=True)
2 Replace the German text Wähle eine Option 
with the text of your choice.
Here’s an example app and the corresponding output:
Code
import streamlit as st
change_text = """
<style>
div.st-cs.st-c5.st-bc.st-ct.st-cu {visibility: hidden;}
div.st-cs.st-c5.st-bc.st-ct.st-cu:before {content: "Wähle eine Option"; visibility: visible;}
</style>
"""
st.markdown(change_text, unsafe_allow_html=True)
options = st.multiselect(
'What are your favorite colors',
['Green', 'Yellow', 'Red', 'Blue'],)
st.write('You selected:', options)
Output
Are you able to reproduce this behavior?
Happy Streamlit-ing! 
Snehan
Inspiration: How can I replace text with CSS?