add translations of 5.2.8, 5.2.9, 5.2.10
Merge github.com:phasetr/math-textbook into translate-5.2.8-
remove English from 5.2.8, 5.2.9
fix Japanese in 5.2.8, 5.2.9
remove English from 5.2.9
translate 5.2.10, 5.2.11, 5.2.12
Improve expressions in both Japanese and English
Merge pull request #3 from phasetr/translate-52x-remains

Improve expressions in both Japanese and English
Merge pull request #13 from esotericaone/translate-5.2.x-remains

Translate 5.2.x remains