Oop- we ran into some trouble here. Here is the error stack trace explanation:

Explanation for issues of type "MissingQuantity":
Different languages have different rules for grammatical agreement with
quantity. In English, for example, the quantity 1 is a special case. We
write "1 book", but for any other quantity we'd write "n books". This
distinction between singular and plural is very common, but other languages
make finer distinctions.
This lint check looks at each translation of a and makes sure that
all the quantity strings considered by the given language are provided by
this translation.
For example, an English translation must provide a string for
quantity="one". Similarly, a Czech translation must provide a string for
quantity="few".

You need to add "plurals". I don't speak german but I guess that some adjectives are changing for the number of items. Read how to deal with plurals in android studio, you should have a warning.
It should be something like this:
<plurals name="n_selected">

PS: I really need to fix the PR running.