Well, if the French want to speak of "the mother Fatherland", that's their business; as we all know,
depending on the time of day... But more seriously, a translation has to take into account not only what the original says, but also the connotations in the target language. The Marseillaise is a martial song urging its listeners to take up arms, enlist, and throw the foreigners coming to rape their women and take their stuff back into the sea. In that context, 'fatherland', in English, makes a lot more sense. 'Motherland' has connotations of birthplace, love of place, amber waves of grain; 'fatherland' speaks of armies, martial virtue, and giving Johnny Foreigner a damn good thrashin'. Never mind French idiom, 'motherland' just does not belong in an English translation of the Marseillaise; it may be linguistically correct but it's poetically wrong. Of course, if you were trying to get the literal denotation of the French words, that would be a different translation altogether; but that's not usually the aim when translating poetry, where you instead try to get the connotations and associations right - sometimes, admittedly, at the cost of some violence to the exact meaning.
A different example: Consider Ibsen's national epic, "Terje Vigen". In Norwegian it opens "Der bodde en underlig gråspreng en, på den ytterste
nøgne ø". This is conventionally translated "There lived a strangely gray-haired one, on the outermost
windblown isle." Now, that 'nøgne' would, if translated literally, become 'naked'; and this would make just as much sense and have the same scansion properties. So why 'windblown'? Because 'naked' in English doesn't have the same connotations, perhaps because not a lot of people in English-speaking countries live on islands exposed to the North Sea wind and know how barren they are. Also, 'naked' in English more usually applies to human rather than geographical bodies; it's not
wrong to use it of an island, but it just feels a tiny bit off. So the translator goes for 'windblown' because this can be more easily used as descriptive of an island without causing the reader to stop, even for a tenth of a second, and say (perhaps below the level of conscious attention) "hang on a minute". And note that this all applies to a word in which the English and the Norwegian are cognates with the same literal meaning! In the same way, 'motherland' isn't
wrong as a translation of 'patrie'; it just isn't quite
right, either. In translating poetry these nuances are important.
I will agree so far as to say that they are worth what you paid for them.