Lakes are generated as a general feature which aren't specific to any biome (vanilla disables them in some biomes by hardcoding in exceptions, instead of using e.g. "if (biome.hasWaterLakes())..."). While I have no experience with Forge, much less 1.12, I found a thread which has a solution; you have to use "PopulateChunkEvent.Populate.EventType.LAKE" in ChunkGeneratorOverworld and cancel it, calling your own code instead (as they provided; add your biomes to the list. See the replies to the thread for details on how to do this).
Alternatively, you could use your own chunk generator class which replaces the vanilla class, which may be easier if you want to change a lot of things (I replaced the entire class in my own mod for this reason). This could cause issues with other biome mods though (if you include any Forge hooks that are in the vanilla class it shouldn't be a problem unless they also replace it, so this is best done for custom dimensions/world types).