This question is technically in breach of the the 20 year rule, as the last two countries to metricate did so in 1998 (Jamaica) and 2005 (St. Lucia). However, pinning down a date is difficult for many countries as many have very long transition period (e.g the USA, where the metric system has been legal since the 1800's) or old units remain co-official in some circumstances (e.g the UK). As the process is still very much ongoing in many countries I won't comment further, only to direct you to /r/ask_politics, /r/asksocialscience or /r/neutralpolitics