Does the indefinite article "an" apply to vowel sounds from foreign languages?

When adding an indefinite article to English nouns, you either select a or an depending on the sound of the first letter of the noun that it precedes.

In some languages vowel sounds occur that do not exist in the English language, like ui in Dutch or ы in Russian. I know Russian doesn't have any words starting with ы, but I would still need an article if I discuss a/an ы vowel.

If I use a foreign noun in English language, like when I explain this word in English, does "an" only occur for vowel sounds that also exist in English, or does "an" apply to all vowels, regardless of its existence in English?

For example, which of the following is correct:
- A uitzondering is an exception.
- An uitzondering is an exception.