diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 291aa5d..0407a6f 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -253,6 +253,13 @@
     rm swift-5.1.4-RELEASE-ubuntu18.04.tar.gz && \
     swift --version
 
+# Locale(s) for cpp unit tests
+RUN apt-get install -y --no-install-recommends \
+`# Locale dependencies` \
+      locales && \
+    locale-gen en_US.UTF-8 && \
+    update-locale
+
 # cppcheck-1.82 has a nasty cpp parser bug, so we're using something newer
 RUN apt-get install -y --no-install-recommends \
 `# Static Code Analysis dependencies` \
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index 9775fbb..a175ed5 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -255,6 +255,13 @@
 #     rm swift-4.2.1-RELEASE-ubuntu18.04.tar.gz && \
 #     swift --version
 
+# Locale(s) for cpp unit tests
+RUN apt-get install -y --no-install-recommends \
+`# Locale dependencies` \
+      locales && \
+    locale-gen en_US.UTF-8 && \
+    update-locale
+
 # cppcheck-1.82 has a nasty cpp parser bug, so we're using something newer
 # don't need this on disco, nobody uses it
 # RUN apt-get install -y --no-install-recommends \
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 62da2c4..1c4373e 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -235,6 +235,13 @@
 # Rust dependencies
 RUN curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.40.0 -y
 
+# Locale(s) for cpp unit tests
+RUN apt-get install -y --no-install-recommends \
+`# Locale dependencies` \
+      locales && \
+    locale-gen en_US.UTF-8 && \
+    update-locale
+
 # Clean up
 RUN rm -rf /var/cache/apt/* && \
     rm -rf /var/lib/apt/lists/* && \