1. It is scientifically correct to assume that the English language has socalled “mixed character”?

Grammatical Assimilation. Usually as soon as words from other languages were introduced into English they lost their former grammatical categories and paradigms and acquired new grammatical categories and paradigms by analogy with other English words.

2. 
   However, there are some words in Modern English that have for centuries retained their foreign inflexions. Thus a considerable group of borrowed nouns, all of them terms or literary words adopted in the 16^th century or later, have preserved their original plural inflexion to this day, e.g. phenomenon (L.) – phenomena, addendum (L.) – addenda, parenthesis (Gr.) – parenthesses. Other borrowings of the same period have two plural forms – the native and the foreign, e.g. vacuum (L.) - vacua, vacuums, virtuoso (It.) – virtuosi, virtuosos. Если коротко, то: All borrowed words undergo the process of assimilation, i.e. they adjust themselves to the phonetic and lexico-grammatical norms of the language. Phonetic assimilation comprises substitution of native sounds and sound combinations for strange ones and for familiar sounds used in a position strange to the English language, as well as shift of stress. Grammatical assimilation finds expression in the change of grammatical categories and paradigms of borrowed words, change of their morphological structure. Lexical assimilation includes changes in semantic structure and the formation of derivatives.
   

11. Lexical Assimilation of borrowings. Historical development of borrowed words semantic structure.