diff --git a/solenv/gbuild/UITest.mk b/solenv/gbuild/UITest.mk
index 42779c8a13ba..67004b29af21 100644
--- a/solenv/gbuild/UITest.mk
+++ b/solenv/gbuild/UITest.mk
@@ -26,7 +26,7 @@ gb_UITest_GDBTRACE := --gdb
 gb_UITest__interactive := $(true)
 endif
 
-gb_UITest_COMMAND := LIBO_LANG=C.UTF-8 $(gb_UITest_EXECUTABLE) $(SRCDIR)/uitest/test_main.py
+gb_UITest_COMMAND := LIBO_LANG=en_US.UTF-8 $(gb_UITest_EXECUTABLE) $(SRCDIR)/uitest/test_main.py
 
 .PHONY : $(call gb_UITest_get_clean_target,%)
 $(call gb_UITest_get_clean_target,%) :