What is common language? How does common language help professionals manage quantities of data efficiently?

Answers will vary. Fractions, ratios, and percentages are examples of common language. In effect, common language reduces difficult figures to the "common denominators" of language and ideas.
Common language also involves the use of indicators other than actual count or quantity. The Dow Jones Industrial Average provides a measure of stock market performance and is certainly easier to understand than the complete New York Stock Exchange figures. Similarly, oil is counted in barrels rather than in the quart or gallon sizes purchased by consumers. Because of inflation, dollars are not accurate items to use as comparisons from one year to another in certain areas. For example, automobile manufacturers use "automobile units" to represent production changes in the industry. The important thing for the report writer to remember is that reports are communication media, and everything possible should be done to make sure communication occurs effectively.