I am only beginning to learn the German language and when encountering the word 'sprachbund' which means 'language alliance', I do not know which type of substantiv it belongs to (accusative, dative, or nominative). According to Wikitionary the word is masculin, the genitive is sprachbundes and plural is sprachbünde. This makes me wonder if it can be all three, would it be possible to have der, den or dem sprachbund?
It seems there is a misconception here as to what grammatical cases are. I infer this from the title, where you speak of "declension class", and from the body, where you speak of "type of substantiv (accusative, dative, nominative)" and where you ask "if it can be all three".
This leads me to think that you believe that each noun was classified to one particular case. But in fact, every noun can have all cases, which is already the answer to your question: Yes, Sprachbund can be all three cases. In other words, you cannot say something like "Sprachbund belongs to the class of accusative nouns".
However, it depends on the noun's function in the sentence, which case it has there. It can be accusative, when its function in a sentence is to be an accusative object. But it can also be dative, namely when its function is to be a dative object. It is nominative, when it is the subject of a sentence. And it can be genitive in other situations. Here are some examples for Sprachbund:
- Der Sprachbund ist eine Gemeinschaft. (nominative)
- Was ist die Aufgabe des Sprachbunds? (genitive)
- Ich bin Mitglied im Sprachbund. (dative)
- Er tritt in den Sprachbund ein. (accusative)
In German, there is nothing usually called 'declension classes', but what comes closest to it are the genders: masculine, feminine, neuter -- but there are many finer subdivisions! They define how a noun is declined.
But what is declension, to begin with? It's assigning a noun a case and a number. Cases are: nominative, genitive, dative, accusative. Numbers are: singular and plural.
Now putting these two parts together: The cases and the numbers stay always the same for all nouns, just how they are actually marked on the noun changes according to the gender subdivision, or, if you want, 'declension class'. With your example Sprachbund, you see that only the genitive (des) Sprachbundes is positively marked, all other cases have no specific ending, however (i.e. they are marked by the 'null morpheme'). That holds true for many masculine nouns, but opposed to that, e.g., the genitive case is unmarked with feminine nouns.
This already has some good answers, but I thought there were a few additional details that might be helpful for learners like myself. German follows what I call the "Right Hand Rules" law, meaning that when you have a compound such as Sprachbund, it's declined according to the last part Bund. Wiktionary does not have a declination table for Sprachbund, but the declination table for Bund works just as well, and Wiktionary does have that.
Wiktionary does list the genitive singular and nominative plural, and with this, and a few not too complex rules that apply to all nouns, you can determine the entire table on your own. German inflection is in general more complex than it is in English, so don't expect the German rules to be too simple, but they aren't that complex either.
The second part Bund, is masculine, so, again by the Right Hand Rules law, Sprachbund is also masculine. Most, but by no means all, masculine nouns form plurals with an -e, and Bund is no exception. Also, like many one-syllable words, Bund adds an umlaut in the plural, so the plural is Bünde, and therefore the plural of Sprachbund is Sprachbünde. Most German nouns form the genitive singular by adding -(e)s, and again Bund is not an exception. So the genitive is Bundes (with Bunds also possible), and so the genitive of Sprachbund is Sprachbundes. What I'm getting at is even without looking it up in a dictionary, you can, with some simple rules of thumb, make a reasonable guess about the plural and genitive, which happens to be correct in this case.
German has about two dozen declension classes for nouns, at least as I understand the term. Most of these only have a few entries though and most nouns fall into a relatively small number of patterns. To me it's more useful to remember the rules of thumb that apply in most if not all cases. There are rules of thumb for forming the nominative plural. Another rule tells you how to use the nominative plural to form all other plural cases. Declining the singular by case is usually straightforward as well, though there are some exceptions which can be tricky. What I'm saying is while it may be possible to define declension classes in German, and this may be an efficient way to store the information on a computer, I don't think useful for learners to decline nouns this way.
By the way, As far as I can tell, Sprachbund is not a common word in German. Apparently, somehow, it was borrowed into English and some other languages as linguistic jargon. But for a typical learner of German I don't see any reason for learning it or its inflections except as an example of how to deal with compound nouns.