The question of whether or not it is "proper" is meaningless, unless you define the particular arbiter of manners who you want to defer to. There is no authority for the English language.

the singular form of the string (note that it can be used for numbers other than one in some languages, so '%s item' should be used instead of 'One item')

However, translating/interpreting is not the only job a polyglot can have. If we speak about a world-language bearer (English, Spanish, Chinese, etc.), the range of possibilities broadens. My uncle has once told me: "You have English, now you can learn anything." The point is, language is a crucial aspect of development both in terms of career and social life. Language(s) is(are) the basis. You move further on learning new knowledge and acquiring new skills with time. Though, they are not limited to the language you know.If you are talented at languages, you can decide which one you are interested in and study it. Once you have decided, you then choose the particular subject field you want to work in. If you are talented "only" at languages, other ways are open to you. You can be a teacher, tutor, again translator/interpreter, etc.All I want to say is language is power. Where I come from, it is a treasure. I do not know what is it like to be a citisen of a large, highly-developed, economically stable, and successful country. Though, I know for sure, if you know and love languages, you will find your place anywhere.

This will of course depend on your perspective, but: beware Finnish and other highly inflected languages. As a grammar nerd, I actually love this stuff. But judging by my colleagues, you won’t.

Entire sentences are also important because in many languages, the declination of some word in a sentence depends on the gender or the number (singular/plural) of another part of the sentence. There are usually more interdependencies between words than in English. The consequence is that asking a translator to translate two half-sentences and then combining these two half-sentences through dumb string concatenation will not work, for many languages, even though it would work for English. That’s why translators need to handle entire sentences.

(there are languages, which have far more than 2 forms)

Our languages are filled with slang and idioms that make no literal sense. If you were to try to tell a non-native English speaker to hold their horses, there’s a good chance they’ll look at you like your head is on backwards.Similarly, you’d probably be confused if a French person told you that they had the cockroach. But that just means that they were feeling depressed.

However, the "etc." cannot be interpreted as a conjunction in formal logic.