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/* && \