add chinese for pages.cn/linux/add-apt-respository.md

pages.cn/common/git-submodule.md

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+# git submodule
+
+> Inspects, updates and manages submodules.
+
+- Install a repository's specified submodules:
+
+`git submodule update --init --recursive`
+
+- Add a git repository as a submodule:
+
+`git submodule add {{repository_url}}`
+
+- Add a git repository as a submodule at the specified directory:
+
+`git submodule add {{repository_url}} {{path/to/directory}}`
+
+- Update every submodule to its latest commit:
+
+`git submodule foreach git pull`