Is the second て here used for the meaning of 'via'? I searched 'なんて', found it is usually to express emotional feelings and may not fits this sentence. But for 'via', shouldn't it be 'で'? Like "大きい声で読んでください" and the previous '英語で', so probably I'm wrong.
And I searched for the usage of last '言うの' but only to find '言うの意味' or something. What's the final 'の' used for?
You're correct to assume that this なんて is not being is used to express emotion. (Actually, there are two different なんて that can convey emotion; one expresses derision or dismissiveness, while the other expresses surprise or admiration.)
In your example, 英語でなんて言うの is just an informal way of saying 英語で何{なん}と言うのですか. In informal speech, the quotative particle と often becomes って. After words that end with ん, it instead becomes て, as explained in 大辞林:
て 〔 助詞「って」が撥音「ん」で終わる語に付く場合に用いられる〕 一 （ 格助 ） 「って」（格助）に同じ。 「これはなん－いうのだろう」 「ぼくは知らん－答えておいた」
If you try to pronounce なんって, it should be clear why this change occurs.
For the sentence-final の, see @KHS's answer to your question, and also, as broccoli face - mask cloth suggested in a comment, the answers to this earlier question.
“ これって英語でなんて言うの”
It is colloquial. In proper (written) Japanese, the sentence could be expressed like "（あなたは）これを英語で何と言います（か）？" ("What would you call it in English?").
これ ”って”（＝を／のことを：of/about） なんて言う（の）? By putting "の" in the end of the sentence, you would add some friendly or casual or childish atmosphere. For instance, if you are talking to young children or your siblings, you may put "の" in the end. (But if it is for an adult stranger, you may sound a bit impolite or over‐familiar.It depends.)
Am answering your question?
