Hello @wlixw87
Here is some info you probably wait for 
(cc @glo and @LuisAlaguna)
We don’t take the relations when using the “fill in from” feature because of a technical limitation. In some cases, depending on the relationship type between 2 entities, and depending on the activation of i18n on only one or both Collection-types (CT), recreating the same relation in several locales could break it.
e.g. In the case where i18n is activated on a CT but not on the other one, and where the relationship type is one-to-one, then if we just recreate the same relation in a new locale we break it on the other side of the relation (the other CT) without knowing it.
We thought about limiting the use of some relationship types in the CTB (content type builder) directly to avoid ending up in this kind of situation. But i18n being a plugin we were too limited and it would have been too complex to implement.
To avoid having the same issue when adding a relation manually from the Edit View, we now apply a filter on the relations you can add, depending on whether i18n is activated on one or both CTs you’ll only be able to create a relation to an entity with the same locale.
That’s also why localization is forced on Relational fields.
The good news is that the new db model in the v4 will solve this technical limitation
I18n helped us identify several limitations in our model and the v4 is here to make Strapi more robust and offer more flexibility.
Feel free if you have any questions.
By the way, talking about relations, we are currently doing some researches on this topic and it would be awesome if you could share some info about your Strapi projects, your use cases, and needs regarding relations in this forum post. It’s gonna help us make the most of the new model in the v4 