Just to give an example, consider this short poem:

https://de.wikisource.org/wiki/Ein_Fichtenbaum_steht_einsam

Now try translating this to a language where the tree names are gendered differently, such that e.g. both are male, or both are female.

That is exactly what happened to the Russian translation:

https://ru.wikisource.org/wiki/%D0%9D%D0%B0_%D1%81%D0%B5%D0%...

In Russian, both words are female.

Yeah, Russian is the one I was thinking of.

Although it's actually more complicated, because there's more than one Russian translation, and this problem was tackled in different ways. Lermontov just did a straightforward translation, changing the implied meaning. Tutchev and Fet both changed the pine to another tree such that the word is male: cedar or oak (in the latter case, this also required changing the described environment in which it grows).