I have stared to talk exclusively in English to my son when he was less than 6 months old. However, we have decided not to move to an English speaking country, so I ended up practically alone.

It's been two years since then (he's 2y7m old now) and he clearly understands (my) English (he does know colors in English, cars, can count in both languages to ten...), although he answers in Slovak. He's also able to translate what I tell to people who don't understand English. After two years, I have to admit that I'm still far from native speaker and seriously consider I cease from speaking English with him, so I don't teach him my broken English. 

I have read answers on questions like [Would it be bad to only speak to a child in a language in which you are not completely fluent?](https://parenting.stackexchange.com/q/1169/1078) and referenced studies, but there is no answer on what happens if I stop teaching the second language, or possibly, at what age it is desirable.

* Will the advantages (cognitive or developed executive functions) we earned persist?
* Would these disappear if I cease from speaking English?
* Are there any negative effects on my son if I continue speaking English (beside the fact I may teach him broken English)?