You're correct; there is no way, through Traits in the default game, to improve yields for specific resources, only improvements (like the Hunnic +1 Production from Pastures) or all of a certain type of resource (like Russian +1 Production from Strategics).
The way this is typically done is to create an invisible building in all of the Civ's cities that does the job for you. A very simple building definition for accomplishing what you're looking for would look like this:
Code:
<Buildings>
<Row>
<Type>BUILDING_FAITH_CULTURE_WHEAT</Type>
<GreatWorkCount>-1</GreatWorkCount> <!-- this makes the building invisible -->
<Cost>-1</Cost> <!-- this makes the building unbuildable -->
<NeverCapture>true</NeverCapture> <!-- so other Civs don't get access to this bonus when conquering your cities -->
</Row>
</Buildings>
<Building_ResourceYieldChanges>
<Row>
<BuildingType>BUILDING_FAITH_CULTURE_WHEAT</BuildingType>
<ResourceType>RESOURCE_WHEAT</ResourceType>
<YieldType>YIELD_CULTURE</YieldType>
<Yield>1</Yield>
</Row>
<Row>
<BuildingType>BUILDING_FAITH_CULTURE_WHEAT</BuildingType>
<ResourceType>RESOURCE_WHEAT</ResourceType>
<YieldType>YIELD_FAITH</YieldType>
<Yield>1</Yield>
</Row>
</Building_ResourceYieldChanges>
(
Note: this code is incomplete and will cause problems, refer to post 4 for the proper, corrected version)
The other part of this is the more complex part: how you give the Civ this building. Because it needs to be dynamically managed (say you lose a City and conquer it back; with just this code, you wouldn't get this invisible building back and thus wouldn't get the benefits in that city), I would recommend doing it with Lua, but as I suspect you've just started modding, that may be a bit much.
Another way of doing it, though I'm not entirely sure if it would work as intended, is to do it through your Civ's trait, using <FreeBuilding> and <FreeBuildingOnConquest>:
Code:
<FreeBuilding>BUILDING_FAITH_CULTURE_WHEAT</FreeBuilding>
<FreeBuildingOnConquest>BUILDING_FAITH_CULTURE_WHEAT</FreeBuildingOnConquest>
This way, the building will be provided for free in all Cities you settle and in all Cities you conquer. Because of its <NeverCapture> field, other Civs won't get it from conquering your Cities.
