It seems from some of the comments on the previous post, that there is some confusion on what diglossia actually is, and after looking into it a bit more, I see that it is with good reason, given the international nature of the input, and the different ways the term has been used historically. It comes to English through the French use of diglossie which is an adaptation, introduced by the Greek-French writer Jean Psichari (Ioannis Psikharis), of the Modern Greek διγλωσσία, which simply means ‘bilingualism.’ The French philologist Auguste Dozon already commented in 1889, in reviewing Psichari’s work, that the only possible rendition of διγλωσσία would be bilinguisme, which he however characterized as a mot barbare, and which did not become current in French until the linguist Antoine Meillet used it in 1917. It is a matter of semantics and technical usage to differentiate between these two words anyways, since bilingua is basically a Latin calque for the Greek διγλωσσία (but one which I personally identify with as significant, as I distance myself from my philologist friends in jest by identifying as a linguaphile - a mixed metaphor if you will). The term was not really used in a technical linguistic sense until 1959 by Charles Ferguson, though this was not broadly accepted as standard. In fact, in 1959 the word ‘diglossia’ was present in unabridged dictionaries (such as Webster’s Third International) with ‘condition of the tongue being bifid’ as its only meaning, while the linguistic term ‘diglossy’ was being used at least by linguists specializing in Greek, for example P. C. Costas, and continued to be so used after the publication of Ferguson’s paper, for example by Robert Browning. It is only in dictionaries published in 2000 or later that the sociolinguistic meaning of ‘diglossia’ can be found. Diglossia has settled into a more or less accepted usage that includes both a pair of languages which are quite genetically unrelated, as well as variations (I avoid 'dialects' because it is more often a literary standard or Dachsprache and a colloquial that form the pair).
Thus diglossia continues to be used ambiguously to refer to a continuum from stable bilingualism and ambilingualism, to code-switching and register change, all of it dependent on the specific languages involved and how discrete from one another they are considered to be (often based on somewhat arbitrary governmental policies concerning what is an 'official language'). I will not pretend to know anything about Finnish, but one of the funniest examples (I am told) is the Helsinki public transportation website which can be accessed in 'Suomeksi' and 'Slangi':
Reittiopas neuvoo perille
Reittiopas ehdottaa reittejä kahden valitsemasi paikan välille pääkaupunkiseudun joukkoliikennettä käyttäen.
Syötä lähtöpaikka ja määränpää tekstikenttiin. Paikka voi olla katuosoite, pysäkki tai paikannimi. Voit myös valita paikat kartalta tai hakemistosta.
Reissugaidi viisi vuotta!
Stadin kielellä joukkoliikennetietoutta tulkkaava Reittioppaan slangiversio juhli vappuna viisivuotissynttäreitään. Lue käyttäjien kommentteja Reissugaidista vuosien varrelta.
Just comparing this brief introduction in the two registers (which manifestly can both be written, though this is probably the exception to the rule) shows that there are numerous differences in word choice and order, and as someone who doesn't know the language, if I saw these two texts and knew nothing else, I would guess they were somewhat related, but certainly not the identical language.
In any case the fundamental definition of a language is significant to this discussion, but this will never be decided definitively because it is dependent on locally significant socio-political realities rather than linguistic principles. For the purpose of this question there should at least be a clear way of distinguishing between communities which extensively use two genetically (in terms of nuclear language family) unrelated manners of speech and communities which switch between closely related manners of speech.
For this it may be worthwhile to use the Ausbausprache - Abstandsprache - Dachsprache continuum which refers to the social relationships among languages. I would characterize simultaneous use of two Ausbausprache (referring to their both being used - though perhaps to differing extents in spoken and written and educational contexts, such that they are each independent), as in Quechua-Spanish in South America, French-English in Quebec, Arabic-Spanish/Latin/Portuguese-Hebrew in Andalusia, Kabyle-Arabic in Algeria, Wolof-French in Senegal, etc. need not be classified as diglossia, since the term stable bilingualism sufficiently covers those situations. An Abstandsprache - Abstandsprache or Abstandsprache -Ausbausprache combination, might debatably describe some of the above, but would be certainly true of Guarani-Portuguese in Brazil (and possibly in Paraguay with Spanish, I am not sure how much Guarani is written and taught there), Pulaar or Soninke - French in Mauritania, Siswati - English in Swaziland, (and undoubtedly many languages in Africa that are not written extensively, alongside either lingua franca like Swahili, or colonial languages). This would be what Joshua Fishman calls "extended diglossia."
However this is problematic, because it doesn't acount for the fact that in a diglossic case like Arabic, you have numerous variants which are Abstand towards one another because they are mutually unintelligible, but they share the Modern Standard Arabic Dachsprache. Thus, they are almost as equally 'extended' in terms of intelligibility as an Abstand - Ausbau pair would be, but they are genetically related. Furthermore, the pairs of "extended diglossia" are qualitatively different than coexistent variants of the same language (using the term broadly). They in fact represent a case of linguistic co-dependence, where the primarily spoken language either does not have the cultural prestige or the commercial utility to merit the study and attention of the written Ausbausprache. Thus the situation should be described as co-dependent bilingualism (which from a sociological perspective does insinuate a certain dysfunctionality, or at least instability which is not true of the cases of "stable bilingualism").
I posit that it is more significant and precise to classify Abstand-Dachsprache cases such as the famous Kathaveroussa-Dimotiki in Greece (before the mid 20th century), Catalan-Spanish in Northern Spain, Schweizedeutsch - Hochdeutsch in Switzerland, Arabic dialects - MS Arabic across North Africa and the Middle East, etc. as diglossia not as a statement that Catalan is not a language (for example), but as a way of preserving the uniqueness of the term as a marker that differentiates itself from bilingualism in the relatively rare cases where the historical occurence of two different manners of speaking reflects diachronous standardizations/fossilizations of language, rather than contact between unrelated languages.