I was specifically thinking of whether the voiceless velar fricative [x] as in Polish could be represented in Japanese, but [x] would be the same or very similar in every language which contains it, right?
Anyway, just wondering. Off the top of my head I can't think of a way, nor find one by perusing the IPA for Japanese article or online books.
I will assume that by "translate" you mean which syllables in words loaned by Japanese correspond to [x] in their source language.
The answer is that words containing [x] which come directly from languages with that phoneme are sometimes rendered ッハ hha. The most common examples would be Bach バッハ Bahha and Mach マッハ Mahha.
Other times, it is simply rendered ハ (cf Zakharov Zaharofu, Halacha ハラーハー), or particularly word-finally, フ (cf Lech Wałęsa Refu Vawensa). Note that while ending not in /x/ but /h/, the Arabic name for God is sometimes transcribed as アッラーフ Arrafu.
Words of Greek origin beginning in Greek with χ, however, tend to have that sound represented instead with syllables from the か行 (k- syllables), in line with English.
Another subtlety is that the German orthographic ch which before high vowels is a palatal fricative [ç] and not a velar [x] is almost universally transcribed as ッヒ hhi, as in アルベリッヒ Aruberihhi for "Alberich".
