Ray Larabie

The fact that fonts in previous centuries had little to no kerning seems irrelevant to how fonts are made today. Those designers were working against certain technical limitations. Technical limitations create style. We're working against a different set of limitations. Thousands of kerning pairs combined hundreds of kerning classes leaves us with almost almost unlimited possibilities.
Language support beyond your likely customer base is there to make localization easier for your customers. Multilingual apps, packaging and instruction manuals are the norm these days. Apps have fluid localization. They often start in one language, gain popularity and languages get added one-at-a-time.

They're important because it's the only way to type Spin̈al Tap properly.

As for Japanese, do some market research to make sure Japanese is appropriate for your product. Sometimes foreign companies try to market their products with localized branding and it's counterproductive. For certain products, English is preferred. For example: watches, phones, automobiles, foreign clothing labels...you rarely see Japanese displayed on those. 

I wonder how strong national preferences for accents are after decades of exposure to system fonts. If it's true the Portuguese readers were once fussy about the design of their cedillas while French readers weren't, is it still true in 2017? Do readers really care or is it just something that typographers prefer? Are readers somewhere truly struggling with comma-like cedillas?

Even if just for exploration, try building a uniform width hairline. Maybe only 10 or 20 units thick. Move the weight axis slider and you might find a pleasing light weight somewhere between 150-250. It will still retain some of the thick/thin of the heavier weight. Sometimes the lightest weight loses too much flavor on the way to hairline but often it produces exactly what I was looking for.