When a localization user uses diff_l10n_branches.py,
the directory of content on 'Proposed Solution' is fixed.
Thus, this commit changes the directory name according to the l10n language.
* Add a helper script for localization
The script generates a report of outdated contents in
`content/<l10n-lang>` directory by comparing two l10n
team milestone branches.
* Add description for scripts/diff_l10n_branches.py