set your Locale in your tests

LocaleRule.java

package fr.barreverte.test;

import org.junit.rules.ExternalResource;

import java.util.Locale;

public class LocaleRule extends ExternalResource {

    private final Locale toRestore;
    private final Locale forTest;

    public LocaleRule(Locale forTest) {
        this.toRestore = Locale.getDefault();
        this.forTest = forTest;
    }

    @Override
    protected void before() throws Throwable {
        Locale.setDefault(forTest);
    }

    @Override
    protected void after() {
        Locale.setDefault(toRestore);
    }
}