The subject of words that Gaelic 'doesn't have' is frequently raised, rather ironically as English is renowned for 'borrowing' words from other languages, like 'double entendre' or 'faux pas'. Like any language, Gaelic has to constantly invent new words for new concepts (like podcasting and blogging), and this is happening all the time. Sometimes Gaelic 'borrows' these words from other languages. It borrowed 'helicopter' and 'television', for example, but not from English - these words are both Greek, and English borrowed them too, as did French, and Spanish, and many other languages.
It is true that many Gaelic speakers do mix Gaelic and English in their speech. This is called 'code-switching' and happens when people associate some experiences or ideas with one language and some with another. Children educated through the medium of Gaelic will in fact have a bigger technical vocabulary than many older speakers, as very few older speakers were lucky enough to receive any of their education through the medium of Gaelic.
One of the joys of learning new languages is the 'untranslatable' words, phrases and concepts which make up part of any language and culture.