I'll go for a pessimistic-realistic view:
Limit the language families your translator can work at.
Here's an example: English, French, Spanish, Danish and 440 other languages are known as the Indo-European language family. They are grouped this way because they use a similar alphabet, similar means of conveying an idea, and share many cognates with each other. When translating in between these languages, you might have to swap a few phrases around, use a different dictionary, but they are otherwise the same. People with a natural tongue in this language are more likely to learn a second language in this category, like Spanish and Dutch.
Languages outside this family, however, like many Asian languages, use a different approach. They have a completely different system of letters. Instead of using characters to convey how to pronounce the word, they use it to convey the meaning of the word.
Translating between these families would be much tougher than intra-family translation. Therefore, you could make a translator that only works in one family.
Decrease the Accuracy of the Translator.
Go to Google Translate. Translate "How many pigs does a dog eat?" to Hebrew. Copy the words, and translate it back to English. At the time of this answer (2016-12-06), I got back, "Some pigs do not eat dog?"
Translators are known today to be inaccurate. Many foreign exchange students in my high school are known to use Google Translate to convert their native language to English, and I hear the teacher talking to them every other week. Translators aren't perfect, and converting possibly extra-terrestrial languages would be with 100% accuracy is neigh-impossible.
Have your device read the conscious of the Translator.
This has a few requirements:
- Somebody invented and patented a technology that can read the raw thoughts of a person.
- Alien and human minds are alike enough for this technology.
- People are okay with their thoughts being read!
By reading the raw thoughts of an entity, it removes all borders of language; except when it has to translate those thoughts into a language. While constructed, logical languages designed to convey raw speech like Lojban are available, the receiving party would have to speak this logical language without error for this translator to be 100% accurate.
This technology would have other ramifications. If this translator is owned by a company or a government, then the users could complain about how this device could be used to read their minds for their own hidden agenda.
What's your choice?
I'd say a mix of the first two.. Intra-family translations will be much more accurate than between language families. Otherwise, it should have the capabilities a little better than Google Translate.