It's quite inaccurate to say the fall of Ringgit value would eliminate all gains, and actual inflation rate much higher than the published one.
For Malaysian residents who invest in Ringgit denominated assets, there's absolutely no currency risk. Why do you bother about the value of Ringgit againts other currencies when you would spend most of it on local products? And if you buy a significant amount of products from abroad, you can only blame yourself for not properly hedging your risk. Foreign currency accounts have been available for quite some time.
On the inaccuracy of the consumer price index (cpi), the common measurement for inflation that's widely used. In the calculation of the CPI, the department of statistics would pick a representative bundle that represent the average consumption for a Malaysian. Every country has different weightage of different items in the bundle. Developing countries like Malaysia usually put food as the biggest weightage in the CPI, followed by transport. For more details of the calculation, you could go to Department of Statistics website.
For any individual, of course the inflation rate is very different from the CPI. Ali may spend more on food, but Muthu may spend more on transportation etc etc.
And A may cook most of his food while B eats frequently outside, while C always go to high end restaurants. Since consumption bundle is different, the inflation rate for different individual would not be the same.
And the CPI is only an estimation for an average Malaysian. There bound to be inaccuracy in estimating data, but no foreign bodies like IMF, World Bank etc have questioned the accuracy of our statistics data, unlike the case with China.