diff --git a/roofit/codegen/CMakeLists.txt b/roofit/codegen/CMakeLists.txt
index 664dace90c5ce..e1fecef63dac2 100644
--- a/roofit/codegen/CMakeLists.txt
+++ b/roofit/codegen/CMakeLists.txt
@@ -34,3 +34,5 @@ if(NOT CMAKE_VERSION VERSION_LESS "3.23.0")
           inc/RooFit/CodegenImpl.h
   )
 endif()
+
+ROOT_ADD_TEST_SUBDIRECTORY(test)
diff --git a/roofit/codegen/src/CodegenImpl.cxx b/roofit/codegen/src/CodegenImpl.cxx
index bc3bdc3556d3e..750f6912bc380 100644
--- a/roofit/codegen/src/CodegenImpl.cxx
+++ b/roofit/codegen/src/CodegenImpl.cxx
@@ -66,6 +66,7 @@
 
 #include <TInterpreter.h>
 
+#include <locale>
 #include <unordered_set>
 
 namespace RooFit::Experimental {
@@ -76,6 +77,9 @@ namespace {
 std::string doubleToString(double val)
 {
    std::stringstream ss;
+   // The formatting must not depend on the global locale: a comma decimal
+   // separator (e.g. from a German locale) would corrupt the generated C++.
+   ss.imbue(std::locale::classic());
    ss << std::setprecision(std::numeric_limits<double>::max_digits10) << val;
    return ss.str();
 }
diff --git a/roofit/codegen/test/CMakeLists.txt b/roofit/codegen/test/CMakeLists.txt
new file mode 100644
index 0000000000000..6ec0ce7f477b1
--- /dev/null
+++ b/roofit/codegen/test/CMakeLists.txt
@@ -0,0 +1 @@
+ROOT_ADD_GTEST(testCodegen testCodegen.cxx LIBRARIES RooFitCore RooFit RooFitCodegen)
diff --git a/roofit/codegen/test/testCodegen.cxx b/roofit/codegen/test/testCodegen.cxx
new file mode 100644
index 0000000000000..e6c38407d7081
--- /dev/null
+++ b/roofit/codegen/test/testCodegen.cxx
@@ -0,0 +1,114 @@
+// Tests for the C++ code that the RooFit codegen backend generates.
+// Author: Jonas Rembser, CERN 2026
+
+#include <RooFit/CodegenContext.h>
+
+#include <RooChebychev.h>
+#include <RooConstVar.h>
+#include <RooGaussian.h>
+#include <RooRealVar.h>
+
+#include <gtest/gtest.h>
+
+#include <locale>
+#include <regex>
+#include <sstream>
+#include <string>
+
+namespace {
+
+/// Formats decimal points as ',' like a German locale does. Such a locale is
+/// not installed on every test machine, so it is built from a custom facet
+/// instead of requested by name. The resulting locale is unnamed, so making it
+/// global does not also switch the C locale that std::strtod() uses.
+struct CommaPunct : std::numpunct<char> {
+   char do_decimal_point() const override { return ','; }
+};
+
+/// Generate the code for `arg` with `loc` as the global locale, restoring the
+/// previous global locale even if code generation throws.
+std::string codeUnderLocale(RooAbsArg &arg, std::locale const &loc)
+{
+   const std::locale old = std::locale::global(loc);
+   std::string code;
+   try {
+      RooFit::Experimental::CodegenContext ctx;
+      ctx.buildFunction(arg);
+      code = ctx.collectedCode();
+   } catch (...) {
+      std::locale::global(old);
+      throw;
+   }
+   std::locale::global(old);
+   return code;
+}
+
+/// Erase the global counter from the generated function name, which differs
+/// between two code generations of the same model.
+std::string normalized(std::string const &code)
+{
+   return std::regex_replace(code, std::regex{"roo_codegen_[0-9]+"}, "roo_codegen_N");
+}
+
+/// How a plain stream formats 0.5 under `loc`, to verify that the facet is
+/// actually in effect (otherwise the tests below would pass vacuously).
+std::string formatWithStream(double val, std::locale const &loc)
+{
+   const std::locale old = std::locale::global(loc);
+   std::stringstream ss;
+   ss << val;
+   std::locale::global(old);
+   return ss.str();
+}
+
+} // namespace
+
+// The generated code is C++ source, so its number formatting must not follow
+// the global locale: under a comma-decimal locale the literals came out as
+// "0,5", which does not compile, and inside a function call argument list the
+// comma even turns one argument into two.
+TEST(RooFitCodegen, ValueLiteralsAreLocaleIndependent)
+{
+   const std::locale comma{std::locale::classic(), new CommaPunct};
+   ASSERT_EQ(formatWithStream(0.5, comma), "0,5");
+
+   RooRealVar x{"x", "x", 0.5, -10, 10};
+   RooRealVar mean{"mean", "mean", 1.25};
+   RooConstVar sigma{"sigma", "sigma", 0.75};
+   RooGaussian gauss{"gauss", "gauss", x, mean, sigma};
+   x.setConstant(true);
+   mean.setConstant(true);
+
+   const std::string code = codeUnderLocale(gauss, comma);
+
+   EXPECT_EQ(normalized(code), normalized(codeUnderLocale(gauss, std::locale::classic())));
+   for (std::string const &literal : {"0.5", "1.25", "0.75"}) {
+      EXPECT_NE(code.find(literal), std::string::npos) << literal << " missing from:\n" << code;
+   }
+   for (std::string const &corrupted : {"0,5", "1,25", "0,75"}) {
+      EXPECT_EQ(code.find(corrupted), std::string::npos) << corrupted << " emitted in:\n" << code;
+   }
+}
+
+// Same for the doubles that the codegen implementations pass to the generated
+// function calls directly (here the observable range of RooChebychev), which
+// are formatted by CodegenContext::buildArg() and not by codegen's
+// doubleToString().
+TEST(RooFitCodegen, CallArgumentsAreLocaleIndependent)
+{
+   const std::locale comma{std::locale::classic(), new CommaPunct};
+
+   RooRealVar x{"x", "x", 0.125, -0.5, 2.25};
+   RooRealVar a1{"a1", "a1", 0.375};
+   RooChebychev cheby{"cheby", "cheby", x, a1};
+
+   const std::string code = codeUnderLocale(cheby, comma);
+
+   EXPECT_EQ(normalized(code), normalized(codeUnderLocale(cheby, std::locale::classic())));
+   for (std::string const &literal : {"-0.5", "2.25", "0.375"}) {
+      EXPECT_NE(code.find(literal), std::string::npos) << literal << " missing from:\n" << code;
+   }
+   for (std::string const &corrupted : {"0,5", "2,25", "0,375"}) {
+      EXPECT_EQ(code.find(corrupted), std::string::npos) << corrupted << " emitted in:\n" << code;
+   }
+}
diff --git a/roofit/roofitcore/inc/RooFit/CodegenContext.h b/roofit/roofitcore/inc/RooFit/CodegenContext.h
index f61b3f08450bb..18e70482c5453 100644
--- a/roofit/roofitcore/inc/RooFit/CodegenContext.h
+++ b/roofit/roofitcore/inc/RooFit/CodegenContext.h
@@ -21,6 +21,7 @@
 
 #include <cstddef>
 #include <iomanip>
+#include <locale>
 #include <sstream>
 #include <string>
 #include <type_traits>
@@ -139,6 +140,7 @@ class CodegenContext {
    std::string buildArg(T x)
    {
       std::stringstream ss;
+      ss.imbue(std::locale::classic()); // the generated code is C++, not locale-dependent text
       ss << std::setprecision(std::numeric_limits<double>::max_digits10) << x;
       return ss.str();
    }
@@ -215,6 +217,7 @@ std::string CodegenContext::buildArgSpanImpl(std::span<const T> arr)
    unsigned int n = arr.size();
    std::string arrName = getTmpVarName();
    std::stringstream ss;
+   ss.imbue(std::locale::classic()); // the generated code is C++, not locale-dependent text
    ss << typeName<T>() << " " << arrName << "[" << n << "] = {";
    for (unsigned int i = 0; i < n; i++) {
       ss << " " << arr[i] << ",";
diff --git a/roofit/roofitcore/src/RooFit/CodegenContext.cxx b/roofit/roofitcore/src/RooFit/CodegenContext.cxx
index 8a1f10e7c958e..72338604602da 100644
--- a/roofit/roofitcore/src/RooFit/CodegenContext.cxx
+++ b/roofit/roofitcore/src/RooFit/CodegenContext.cxx
@@ -22,6 +22,7 @@
 #include <cctype>
 #include <charconv>
 #include <fstream>
+#include <locale>
 #include <type_traits>
 #include <unordered_map>
 
@@ -285,6 +286,7 @@ std::string CodegenContext::buildArg(std::span<const double> arr)
 CodegenContext::ScopeRAII::ScopeRAII(RooAbsArg const *arg, CodegenContext &ctx) : _ctx(ctx), _arg(arg)
 {
    std::ostringstream os;
+   os.imbue(std::locale::classic()); // the generated code is C++, not locale-dependent text
    Option_t *opts = nullptr;
    arg->printStream(os, _arg->defaultPrintContents(opts), _arg->defaultPrintStyle(opts));
    _fn = os.str();