If it has to be official then ‘Is your company's official address still Helsinki?’
It doesn’t matter that ‘The headquarters of the United Nations in New York City’ is wrong.
‘The headquarters of the United Nations is (in) New York City’ is perfect.
There would be an element of guesswork even if the question involved English alone, yet it looks as though by ‘a company's home area’ you mean wherever the head or registered office is situated. (Please remember: the difference between head and registered offices in grammar is irrelevant.)
Acknowledging Finnish as the least usual language in Europe doesn’t alter the fact that it’s extremely unlikely Finnish really doesn’t have the concept ‘state’. City, town, or named urban area are hardly the same things in any language I’ve ever heard.
Even Wikipedia (https://en.wikipedia.org/wiki/State_(polity)) understands that A state is an organized political entity living under a single government. the idea that Finland has no such concept would be inconceivable, unless Finland had been somehow isolated from the rest of the world until last week…
If Finland has neither single word nor simple phrase for ‘state’, that’s one thing. If even ancient, let alone modern Finnish really had no concept of state but only city, town, or urban area that would be a very interesting characteristic indeed.
Are you really suggesting that Finnish recognises nothing less than city, town, or urban area? No parishes, districts nor, in the US American sense, counties? Are you really suggesting that Finnish recognises nothing but a house, in a street, in a city, town, or urban area, and that’s it? That’s conceivable only in two ways.
First, you could insist that Finnish doesn’t recognise things like cinema or radio, television or computers, automobiles, refrigerators or fax machines because there are no words in pure, ancient Finnish for those things…
Second, you could insist that Finnish people are not capable of understanding those terms in other languages and so are unable to incorporate them into Finnish.
That L’Academie Francaise is alleged to have ‘outlawed’ terms like ‘le weekend’ is wholly irrelevant to whether or not Jean Grenouille ‘has the concept of state.’
There are only two ways ‘The headquarters of the United Nations in New York City’ could be acceptable in English and neither is part of normal speech.
First, if it really was a direct translation, perfect in every detail, and described as such at the time.
Second, if it didn’t even try to present itself as ‘proper speech’ but was used, for instance, as a caption for a photograph. In that style the entire caption would take the place of a single noun and so not need to follow any rules but those applying to nouns; not the general rules of the whole language.
Either way, ‘The headquarters of the United Nations in New York City’ is not a complete English sentence. As such either it is excused from pretty-much all rules, or it is simply wrong or both.
‘Locality/hometown/domicile/residency’ might not quite be grammatically wrong but none of them has any real place here; semantic or idiomatic. The terms simply do not apply.
‘Home area’ is not meaningless but neither is it ever used; in British English, anyway.
‘Home office’ is very much a US American term, for which the British equivalent is ‘head office.’
Broadly a local office is a branch of an area within a region within potentially a few other levels within a corporation or business or company but specifically ‘Locality’ applies to a particular office, not to the company as a whole. That’s doubly true in the US where, for instance, unions have ‘locals’ or ‘local chapters’ which in Britain would always be called ‘branches’ or just possibly, ‘chapters.’
‘Hometown’ might work as a geographical term but it is not used for a business; only ever for a family or an individual person.
‘Domicile’ almost exclusively refers to the personal residence of a family or an individual person. ’Residence’ is almost interchangeable with ‘domicile.’
‘Residency’ is a special term reserved for various high officials of government, such as viceroys, governors or ambassadors and even then it sounds archaic. Even in the context of a business which effectively owned a country, such as the East India or Hudson’s Bay Companies, neither the head office housing the board of directors, nor the personal residence of even the most senior of those directors, could truly be termed a ‘residency’