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You can change the whole UI language, but I am not aware of any way to just add a secondary keyboard and not change the language of the system. To change the UI language itself you can follow the instructions Tesla has below.
Touch Controls > Display to adjust display settings to suit your preferences:
Appearance: Customize the display to be Dark or Light. When set to Auto, the brightness changes automatically based on ambient lighting conditions.
Brightness: Drag the slider to manually control the brightness level. If Display Mode is set to Auto, the touchscreen further adjusts based on both the ambient lighting conditions and your brightness preference. Model 3 remembers your chosen brightness preference and adjusts the touchscreen accordingly.
Screen Clean Mode: When enabled, your touchscreen darkens and temporarily disables to facilitate cleaning. Follow the onscreen instructions to exit Screen Clean Mode.
Touchscreen Language: Select the language that the touchscreen displays.
Note
Model 3 must be in Park to change the language. When you change the language, you experience a brief delay as Model 3 shuts down and restarts the touchscreen.
Voice Recognition Language: Choose the language to be used for voice commands.
Voice Navigation Language: Choose the language that the navigation system uses for spoken instructions.
Note
For languages that require a download, select the language in the dropdown list to initiate the download (Wi-Fi connection required).
Text size: Select between Standard and Large to customize the text size on your vehicle's touchscreen.
Time: Choose to display time in either 12 or 24 hour format.
Region Format (if available): Choose a region to define the formatting convention used to display dates (mm dd yy/dd-mm-yy, etc.) and decimal separators (5.123, 5,123, etc.).
Energy Display: Choose to display remaining energy and charging units as either a percentage of battery energy remaining, or as an estimate of the distance you can drive.
I dont need to use those characters so I dont know. If you dont see them on the Keyboard when cycling through it, probably not (without changing the entire UI language anyway).
On my phone keyboard, holding down the letter "e" will give a choice of multiple variants of "e" with umlaut, various accents, circumflexes, tilde, dots and others. Not sure if the Tesla keyboard is setup for that.