At the moment Aspose.Words requires picture format switches in merge fields to be in the US (InvariantCulture) format.
This is because Aspose.Words uses standard .NET functions to format values. Yes, the standard .NET functions require InvariantCulture format strings. The output will be different of course depending on the current culture on the machine, but the input format string must be in the InvariantCulture.
Since Aspose.Words relies on these .NET functions it therefore works this way - requires format switches to follow the InvariantCulture.
Of course it would be nice to allow Aspose.Words to properly handle format switches in all cultures, but this will be a significant effort for us which we are not prepared to do right now. Also, as Vladimir correctly pointed out earlier - if Aspose.Words understood format switches in different cultures - you would have to set the proper culture before executing mail merge. Aspose.Words will probably not be able to detect a proper culture. It will also not be easy to detect the proper culture for you. It is good if you can be sure all documents will come in certain culture, but what if they come in different cultures?
My thinking is that Microsoft making format switches different for different cultures was a design mistake. Basically this is localizing too much, localizing things that should not be localized. Another great (bad) example is localizing names of fields or names of functions in MS Excel. What do you think of a function AVG becoming СРЗНАЧ in Russian version? Same goes for format switches.
The best option for you is to go through your documents and change format switches to use InvariantCulture format. In fact, you can write a program in Aspose.Words to go and do that for you automatically.