The problem here is that when you publish a widget into Portal its hosted outside the WAB and you run into issues with paths. Widgets/MyWidget will no longer work. I'm not 100% sure on i18n, it always seems to work differently with paths to other dojo require items. Anyway I ran into the same issue with the AddData widget so here is what I think. With the geoprocessing widget I see:
RecordSetEditorChooser.js: 26
'dojo/i18n!../nls/strings',
Here there is a path and it is failing to find it, the error message is misleading I think its actually looking for it in the WAB. There should be a way to make it work as a relative path but I'm not sure what it is. I ended up just requiring the js file relatively (so no i18n):
'./nls/strings',
then using the .root property:
RecordSetEditorChooser.js: 38
lang.mixin(this.nls, gpNls.root);
This means your localisation won't work, but at least the widget may work...
With the add data widget there were lots of data-dojo-type within the templates. Remember "widgets" no longer works as a path, this is looking in the widgets folder of the WAB, not local to your widget. To fix this I ended up removing the data-dojo-type and creating the widgets within widget.js. Hoepfully you won't have to do this and it will be the simple one line fix above (because this is a nightmare)
data-dojo-type="widgets/AddData/search/ScopeOptions"
The main point is that you have to get into the mindset that everything must work relatively if you are going to host the widget for portal
