A Japanese speaker can learn to speak German, but that's not the same as a loan word. Japanese is a great example for how loan words are modified to conform to the phonemic structure of a language because it absorbed so much english in the past century.
"Fight" is a word in english, but the loan word in japanese is pronounced "faito" because the language demands that all words end in either a vowel or syllabic 'N'. Japanese people are capable of saying english words without ending them in a vowel, but then they're speaking english, not japanese.
The word for salmon roe in japanese is 'ikura'. This is a loan word from russian: 'ikra' for caviar. But because of the Consonant-Vowel structure of the language, a 'u' was added when the word was borrowed.
One of the funniest examples is when a word is borrowed by japanese speakers and then gets translated back into english, the translators won't always return it to the original form. The name Lily would be pronounced "Riri" in japanese (japanese speakers might not even notice the difference between R and L because they're the same phoneme), and when it's translated back into english, it might come back as "Really". This has been a source of consternation for video game players before.
Likewise, when american english speakers borrow a word from a language with a trill or rolling R, they make it conform to the phonemic structure of the language by changing to a retroflex R.
And these sorts of examples are found everywhere. Every language has a particular structure for how valid words can be formed, and speakers of the language will modify foreign words to fit the sounds they're trained to emit unless they're consciously trying to speak a different language.