Backport PR matplotlib#27581: CI: install German lanugage packs an ubuntu test runners

Merge pull request matplotlib#27581 from tacaswell/ci/de_locale

CI: install German lanugage packs an ubuntu test runners
(cherry picked from commit 4df729a)

Author: dstansby

.github/workflows/tests.yml

@@ -118,6 +118,7 @@ jobs:
               gir1.2-gtk-3.0 \
               graphviz \
               inkscape \
+              language-pack-de \
               lcov \
               libcairo2 \
               libcairo2-dev \

azure-pipelines.yml

@@ -101,6 +101,7 @@ stages:
               gir1.2-gtk-3.0 \
               graphviz \
               inkscape \
+              language-pack-de \
               libcairo2 \
               libgirepository-1.0-1 \
               lmodern \