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To the best of my knowledge the font of a text block is defined in a .gui file and can not be changed in localisation (with the exception of the colour as you mention because that does not as such change the font, only the presentation).

However I will be quick to admit, that since I have never had a use for bold because of the many colours, I have never done any serious work to find out if it is possible, but the way the interface is coded suggests that it is not possible.