The easy fix, whether directly custom formatting a cell or using TEXT(), is to use a country code for a language you know the proper formatting codes for.
For instance, I am in the US, have a US version of Excel, and am familiar with its date code formats. So I'd want to use them and to ensure they "come out" regardless of anyone's Windows or Excel version, or the country they are in, I'd do it like the following (for TEXT(), let's say, but it'd be the same idea in custom formatting):
The function would collect the value in A1, ask Excel to treat it as a date, Excel would and would say fine, it's cool (i.e.: the value is, say, 43857 and not "horse") because it is a positive number which is a requirement for anything to be treated as a date, and let the function move on to rendering it as a date in the manner prescribed. Rather than giving an
#ERROR! as it would for "horse" or -6.
The function would then read the formatting string and see the language code. It would then drop the usual set of formatting codes it loaded upon starting up and load in the formatting codes for English ("en") and in particular, US English ("US"). The rest of the string uses codes from that set so it would interpret them properly and send an appropriate string back to TEXT() for it to display in the cell (and pass on to other formulas if such exist).
I have no way to test the following, but I assume that if one were to use a format that displayed day of the week names or month names, they would be from the same language set. In other words, Excel would not think that even though you specified a country and language that you still wanted, say, Dutch or Congolese month names. So that kind of thing would still need addressed, but would be an easy fix too just involving, say, a simple lookup one could add though it'd be "fun" setting up the lookup table for each language one wanted to accomodate...
However, the basic issue that arises with this problem in general, is very, very easily solved with the country codes. They aren't even hard or arcane anymore now that the [$-409] syntax has been replaced with things like [$-en-us] and [$-he-IL] and so on.