Learn English – One verb to replace ‘be a member of’ a club

I worked at a library. Depending on the job, librarian could still fit, even if the person did not have a specialized degree.

M-W defines librarian as:

a specialist in the care or management of a library

The key term here is 'specialist'. People who serve a specialized role in the library, like doing research or managing a department, would commonly be called librarians. Similarly, the head of a school library would also be a librarian, regardless of their degree.

Someone like me, whose primary job was to put books back on the shelves and man the check out desk, would more commonly be called a library assistant or simply library staff.

Generally, English does not have an equivalent term to Senpai, although since the Michael Crichton book Rising Sun, and its film adaptation, it is an increasingly understood term.

We do not formally acknowledge the same sort of relationship. (As your dictionary suggests, we would just highlight that they joined the company a few years ahead . . .)

So, the short answer to this question is: No, there is no equivalent to the word in English.

There are situations, however, where you may need to address a superior.

In English, if someone is your superior it is always polite to call them Sir or Ma'am, although this may be extremely formal for most situations (especially if you are not in the military). It would generally be reserved for situations where there is a major disparity in level. (A conversation between an employee and the CEO in the mail room, for example.)

Otherwise, to show respect for someone in a senior position to yourself, you would refer to them as Mr., Mrs., Ms., Miss, or Dr. followed by their last name.

The superior may choose to either refer to you in the same manner, or, to call you by your first name.

Another situation which may arise is when someone has a title already:

For example: If John Smith holds the title of General in the army, you can always address him as General Smith, General, or even Sir.

The same holds for members of the clergy. You can call them Father, Reverend, Rabbi, or whatever their title commands. The clergy often allow you to use their title with their first name (e.g. Father John), but it is also correct to use their last name with their title.