The first question that you asked turns out to be more complicated than I think you expected. It appears that there is no word frequency data available through Mathematica, evaluate the following function in Mathematica to see the list of properties that WordData will give access to: WordData[“Properties”]. So, there is no simple explanation that someone can give you, below I offer a suggestion about how I would go about accomplishing this task.

Here is how to figure out where the info comes from in Wolfram|Alpha. At the bottom left of a Wolfram|Alpha page, such as this one, there is a link called “sources”. If you click “sources”, then you will see another link called “word data”. If you click “word data”, then you will see the following citation: “Leech, G., P. Rayson, and A. Wilson. Word Frequencies in Written and Spoken English: Based on the British National Corpus. Pearson ESL, 2001.” Following that citation brings you to this page. which contains the datasets that are presumably used to present the information in Wolfram|Alpha. You can download those datasets and import them into Mathematica using Import[], but it seems that they are not built into the Wolfram Language at this time.

Alternatively, you can also download Google’s ngrams datasets to gain information about word frequency:.

Once you have one of these datasets imported into Mathematica, then you can certainly write a function to search for words with increasing usage and decreasing usage. This is the more complex task of picking trends from a noisy dataset (moving average, linear regression, …), but Mathematica is great for this type of work. See the documentation on Statistical Data Analysis for a good starting point. You can also use Financial Data functions such as DateListPlot[] and LinearModelFit[] to figure out whether a word frequency is trending up or down.

Hope this helps :)