When my iMac enters the lock screen, my default language for the buttons is German; but the language of the day of week displayed in the upper right corner is English. Is there a way to change this?
You may want to check this article from Apple's website.
OS X Yosemite: Customize formats to display dates, times, and more
Change the language and formats used to display dates, times, numbers, and currencies in Finder windows and apps, such as Mail. For example, if the region for your Mac is set to United States, but the format language is set to French, then dates in Finder windows and email messages appear in French.
Tip: If you just want to change how time is shown in the menu bar, select the “Time format” checkbox in the Language & Region preference pane, or select the 24-hour option in the Clock pane of Date & Time preferences. To show the date in the menu bar, also select “Show Date” in the Clock pane.
Choose Apple menu > System Preferences, then click Language & Region.
Choose a geographic region from the Region pop-up menu, to use the region’s date, time, number, and currency formats.
To customize the formats or change the language used to display them, click Advanced, then set options.
In the General pane, you can choose the language to use for showing dates, times, and numbers, and set formats for numbers, currency, and measurements.
In the Dates and Times panes, you can type in the Short, Medium, Long, and Full fields, and rearrange or delete elements. You can also drag new elements, such as Quarter or Milliseconds, into the fields. If you’re not using a 24-hour clock, you can replace AM and PM with whatever you type in those fields.
When you’re done customizing formats, click OK.
The region name in Language & Region preferences now includes “Custom” to indicate that you customized formats.
Your formats are visible in Finder windows and in some apps. For example, dates in Mail or Notes show your custom format.
To undo all of your changes for a region, choose the region again from the Region pop-up menu. To undo just your changes to advanced options, click Advanced, click the pane where you want to undo your changes, then click Restore Defaults.