I really don't know if it's worth to change something here. Pedantry...

Steps to reproduce the issue

• Install J3 and set language to de-DE for install process.
• In databse settings fill all fields but clear the table prefix field.
• Try to go to next installation step.

You'll see an alert (language string INSTL_DATABASE_PREFIX_MSG) that says:

Der Tabellenpräfix muss mit einem Buchstaben beginnen, danach können optional alphanumerische Zeichen und ein Unterstrich folgen.

(EN counterpart:
The table prefix must start with a letter, be followed by optional alphanumeric characters and by an underscore
)

• At least the German text suggests that the underscore is optional, too.