When a foreign word or phrase becomes English [duplicate]

Solution 1:

Words of this nature that are borrowed from other languages are called Inkhorn terms, and was widespread during the mid-16th to the mid-17th century, during the transition from Middle English to Modern English. According to the wiki article, new words were being introduced into the language by writers, often self-consciously borrowing from Classical literature. Critics regarded these words as useless, usually requiring knowledge of Latin or Greek to be understood. They also contended that there were words with identical meaning already in English. Some of the terms introduced, however, filled a semantic gap in English (often technical and scientific words), whereas others coexisted with native (Germanic) words with the same or similar meanings, and often supplanted them. This would explain words like rendezvous.

Another possible explanation (in sociolinguistic circles) for the introduction of loaned terms into a language is prestige, which describes the level of respect accorded to a language or dialect as compared to that of other languages or dialects in a speech community. According to the wiki article, the concept of prestige is closely related to the idea of the standard language, in that the most prestigious dialect is likely to be considered the standard language, though there are some notable exceptions to this rule, such as Arabic. It goes on to say that prestige is particularly visible in situations where two or more languages come in contact, and in diverse, socially stratified urban environments, in which there are likely to be speakers of different languages or dialects interacting frequently. This would probably explain expressions like hasta la vista.

Solution 2:

A word does not have to be in an English dictionary to be considered English. Apart from common words, it is up to the lexicographers' discretion as to what belongs in a dictionary and what doesn't. There are many low-frequency words that may take several decades to be main-stream enough to be considered (by lexicographers). It's easy to pick examples from various domains. From the internet, we have "online", "email", "blog", "login", etc. From electronics, we have "permittivity", "dielectric", etc.

Having said that, coming to the examples you mentioned, "deja vu" and "kosher" have no equivalents that are "more English". Such words are more readily absorbed into English. Words that originate from religion and culture ("kosher") are assimilated most easily because they refer to unique concepts.

"Hasta la vista": I'm not from the US, and so, not all that used to Spanish phrases. I heard it for the first time only in Terminator 2. "See you later" suited my needs just fine.

"c'est la vie": There are enough equivalents that are "more English": "such is life" or "life's like that" or "that's the way the cookie crumbles" and so on.