Anyway, as all fans of the Doctor know the Tardis does the translating for them (and the viewer), unnoticed by everyone, though very occasionally, for the sake of the plot, or more often for the sake of a bit of humour, characters do notice and comment on it.
It is of course a problem in many different science fiction series - the need for the characters to understand each other and the need for the audience to understand all of it. Star Trek managed to visit a different planet every week without communications difficulty thanks to the never-adequately-explained Universal Translator. This device not only managed to translate alien speech from previously unheard of races into whatever passes for English in the future and vice-versa but also, flawlessly, into modern American speech for the audience. A pretty nifty device that only ever malfunctioned when it was necessary to the plot.
Stargate, by comparison, takes an odd and inexplicable approach of having all the races supposedly speaking variations on ancient Earth languages* and taking along an expert who is, (like Universal Translator failures) only ever used when the story needs it. There are token nods towards different races speaking different languages but by and large the team (SG-1) all seem to understand everything said by everyone, wherever they happen to live in the Galaxy.
Buffy and chums, on the other hand, never get to go to alien worlds but do get to beat up lots of demons from other dimensions. There are heaps of references to the demons' own languages. Reference books are always written in mysterious and arcane languages that Giles or Wesley have to study and translate. But, and here's the important bit, whenever it's necessary to the plot, the demons speak English.
I think I see a pattern developing. Translation as a plot device.
What about other series, then? Well one of the favourites of my youth Blake's Seven, avoided the problem by just never mentioning it. Everybody, everywhere did speak English. Going back further to my childhood, the Tomorrow People were all telepaths and, of course, that side steps the problem by having all telepaths able to understand each other.
Sliders, another personal favourite, deals with it by setting every episode not just on alternate Earths, not just in an Alternate America but actually (and very specifically) in alternate versions of Los Angeles and San Francisco. That gets round the accents problem as well.
Battlestar Galactica has fleeing humans from twelve colonies who all have their own languages which we almost never hear because they all also speak a common language which - give or take the odd bit of vocabulary - is unsurprisingly similar to modern American. I'm not sure whether the Cylons they are fleeing have a labguage at all, I don't recall ever hearing one.
So, various ways of getting round the problem have been tried in various programs. Personally I think you might as well take the Blake's Seven approach and just ignore it. The same applies to television and films set in other countries or other times. They are, after all, just for entertainment.
On the other hand I wouldn't mind one of those Universal Translators when I visit other countries. Real life is never quite as convenient as the movies.
(*because they all came originally from Earth)