I am specializing in English and German translations.
I am specializing in English and German translation.
Are both sentences usable? Would the plural of the word highlighted modify the meaning of the sentence?
Translation can be countable or uncountable.
In its uncountable form (which you’ve used in the second sentence), it is the process of translating a text.
In its countable form (used in the first sentence), it is the result of the translation process – that is, it’s the text rendered in another language.
Given those two meanings, both sentences are meaningful. They have different meanings and one of them (the second one) is likely more common.
To specialize in English and German translation is to specialize in translating texts into, or from English and German. Note, it does not mean that you specialize in translating from English to German. To say that, you could try something like, “I specialize in English to German translation.”
To specialize in English and German translations is to specialize in the study of texts that have been translated (by someone else) into English and German. This could be the specialization of someone who studies literature. Instead of studying literature written in English, they study the literature originally written in some other language is translated into English. I don’t know if any such people exist, but they’re probably out there somewhere.