Redmine

Currently, the initials avatar extracts characters based on the `USER_FORMATS` configuration. However, there is a need to apply the initials avatar generation differently depending on the cultural context.

In Korea, a few surnames (like Kim, Lee, Park) are extremely common, making them less useful for distinguishing individuals. Instead, first names are much more diverse and are primarily used to identify
people in daily life. Furthermore, most Korean first names consist of two characters.

Therefore, for Korean users, displaying the "first two letters of the firstname" rather than "one letter of the surname + one letter of the firstname" makes it significantly easier to identify individuals visually.

This patch updates the `User#initials` method to use the first two characters of the firstname when the Redmine environment language is set to Korean (`ko`).

I would appreciate it if you could review and apply this patch.