Certain subjects at university require language skills, simply because you must be able to read the sources. Latin&Greek for theology is widely known, but French&Latin for history f.e., English for psychology aso.....
Some universities lowered this standards due to the fact that many beginners simply and sadly enough do not know the required languages. Honestly i do not know how british universities deal with this, but surely not everything can be ignored or standards lowered.
Top universities in the US generally recommend at least two years of High School-level foreign language education, but that is very far from fluency or proficiency, which is normally not required for university admission. Some High School students take AP exams in foreign languages, as they may also take AP Calculus or AP Physics, but that is not mandatory.
In the UK, many students take a foreign language course for their GCSEs (the exams they take at age 16 as explained before by other posters). I don't know for sure, but I suspect William and Kate must have taken one language at that level too.
For A-levels (the exams students in England and Wales take at age 18), only three or four subjects are studied (but in greater depth). Foreign languages at A-level are not required for admission into most university degrees, unless you want to study a subject where a foreign language is strictly mandatory. For an engineering degree for example, most universities require A-Level Maths and Physics, and a third subject, preferably Further Maths (recommended) or another Science like Chemistry (no foreign language is needed then).
An alternative route to university admission in the English-speaking countries may be, however, the IB which Princess Elisabeth, Duchess of Brabant took and the Princess of Asturias will take from the fall of 2021. For the IB diploma, two languages (normally English and one additional language) are required, plus Math, one natural Science, one Social Science or Humanities subject, and a sixth subject which may be from the Arts group or a second subject from one of the previous groups. But again, UK universities will not consider your second language course for admission unless required for your intended degree; for example, an IB applicant who wants to study engineering will be expected to offer Mathematics HL and Physics HL plus a third Science or Math course at HL, and to have a minimum overall score in the combined six subjects.
EDIT: Canada is not much different. I checked admission requirements for the University of Toronto for example and, for engineering, they mentioned two Math classes (Pre-Calculus and Calculus) in Grade 12, plus Physics, Chemistry and English; a minimum number of foreign language credits may be required, however, for the High School diploma or equvalent in some provinces (?), but, again, far from proficiency.
I have a Ph.D in History and speak one language and one language only ... and I have had no issues because I don't speak any other language. I had no problems getting sources translated if I needed to do so.
The university where I did my undergraduate degree only offered Italian and French and only about 50 students studied either language in the first year and about half that in the second and even fewer in the final year.
The school where I teach has one language - Japanese and this year we have 15 students in Year 8, none in Year 9, 6 in Year 10 and none in Years 11 or 12 and it won't hold back any of these students from getting Ph.D's in any subject other than languages.
Yes, but that is a peculiarity of English-speaking countries. In Latin America, where the OP is from, many Ph.D. programs in History still require two foreign languages. The requirement is not very strict though. In Brazil, for example, I think you only have to take a translation test in the two chosen languages (without a dictionary, however) and only academic texts are chosen for the test. You are not tested for example in oral or writing skills, or in formal grammar and usage.