I like this feature. Many Parameters are often duplicated between translations.
@butaca, it would be interesting to pounder this with possible code/break unwanted side effect.
The one side effect I see is that current Hugo project testing if a Param is set in a language with isset or with will now test it positive no matter what and return the value of the param set in one of the translations.
Also any projects used to use .Param to fallback to the .Site.Param value when the Front Matter param is not set will also find it set but with one of the translations’ value which may not be what they want.
As for which Param value to retrieve when current translation has not param set but more than one of its translations have a competing param set, I think it should return the param whose language has more weight. Same rule already applies to merged Page Resources.
I don’t think all of this would affect too many installs, but this means putting a warning during build and possible noise in the discourse.