Add internationalization support to stacked chart wrapper legend text

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What: Add legendAverageText and legendMaxText props to the stacked chart wrapper to provide internationalization support.

How: Basically, do the same thing as this MR: !16832 (merged), which is for the time series wrapper.

🔴 Note 🔴 This issue can only be worked on once the wrapper is ready.

Internationalization support for chart legend issue: gitlab-ui#245 (closed)