Hello Bruno Michels,
This has been logged as a bug. It is related to a problem that in multilingual mode Sitefinity doesn't set correctly the current UI culture during the installation of Modules.
As a workaround i propose to change the beginning of your module install method like this:
System.Threading.Thread.CurrentThread.CurrentUICulture = CultureInfo.GetCultureInfo (Config.Get<ResourcesConfig>().DefaultBackendCulture.UICulture);
Anyway this bug will be fixed for SP1 that will be releasing this week.
All the best,
the Telerik team
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