Raising bilingual children is like teaching your daughter to sew.

When there is something easy to sew, or something that doesn't show, you might let her do the job. However, when you start to work on something more difficult or important, or something that would show, you might tell her to let you take over. The dress would come out fine, but her sewing skills would be stymied.

The same thing would happen with a child’s speech. If you discuss all the easy or less important things in your native tongue, while saving the important things for the local language, then the child will quickly understand which things are important. You will indeed get your point across on those important things, but the child’s language skills in the mother tongue will be compromised. You got your point across, but your technique is educationally unsound and defeating in the long run.