GUI translation works like this, usually. You have a list of strings/phrases occuring in the GUI and a translation table for each language that assigns the english string to its translated equivalent. With the help of a dictionary (general and domain specific parts) you can even do automatic translations.

But the devil is in the details: Most english speakers don't know foreign languages, so they are unaware of lots of problems that occur from just a word list. E.g. there is a difference between "end" as a noun and "end" as a verb. In German, one would translate the former as "Ende", the latter as "beenden". So for the string table, the programmer would need two different annotated entries "end (n)" and "end (v)" for the translation to be correct. But usually, the translation team only gets "end" without context, annotations or anything, and of course all occurences of "end" will be conflated into one line in the translation table. There are other frequent problems, like sentences containing numbers that require logic to get right and distinguish between Singular/Plural, nothing and maybe even Dual cases. Or the problem that designers got their fingers into a GUI, leaving just enough room for English language strings, but the translations will be cut off or awkwardly skew the layout because they are too short/overflowing/unaligned.

This frequently leads to the situation where as a German, I have to translate to English and back to understand the meaning. Which is why I nowadays avoid translated software and do everything in English. I think translation without lots of effort is pointless. And nobody will invest any significant effort into "small overseas markets".

In FOSS it may be easier to get good translations because of the feedback from real users. They are often more proficient in the target language (than paid translators) and can recognize awkward phrasings and misleading situations. Translation is also an opportunity for small incremental improvements by tons of otherwise non-technical volunteers.

But for the aforementioned reasons, automatic translation makes things worse, not better. I have yet to find a useful translation of technical content, i.e. anything that goes beyond just understanding the general topic of a newspaper article.