If I delete the English language pack and you have another language, what happens? can I safely delete?
Just make the other language as default.
How can I put the other default?
ACP > Configuration > general settings
you can also search for "fdefault language" in the configuration tab
And can I delete the part where it says the footer to change the language and also part of the options?
Yes. in the same settings group there is a yes/no radio button to enable/disable the language selection menu.