Fix: notify service i18n for MR and Issues

What does this MR do and why?

Before this MR, some notification service emails only have English in the content. Such as

Merge request [link] was merged!.

Some files has localization features like: merge_when_pipe_line_succeeds

To align with other notification services, I think we should implement localization for all of them.

This MR only contains a proportion of files that were not localized, just want to minimize the MR.

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