different language for own field-titles?

for example you have a placeholder "_UE_MEMBERSINCE" for the field "member since"

can I define them to for own fields?
Problem is, that I give my fields a title, but cannot react on language-settings

Yes, you can translate your field titles 1 for 1. So if your field title is "This is my field" you can use the below language override.

Key: This is my field
String: This is my field too

You can also make up your own language keys as well. For example MY_FIELD would be as follows.

Key: MY_FIELD
String: This is my field

See the below regarding language overrides.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived