The only reason 'enig' is sometimes written as 'enige' is because of the rules for inflection of adjectives.
But 'enig' is a word with a few different meanings:
It means something like 'cute' in sentences like:
- Wat een enig hondje! (What a cute little dog!)
It means 'only' in sentences like:
- Dit zijn de enige kleren die ik heb (These are the only clothes I have)
And it can mean 'some' in sentences like:
- Enig medelijden, alsjeblieft! (Some empathy, please!)
So 'Hij is de enige jongen' is in the second category while 'Spreekt hij enig Engels?' is in the third one.
As a side note: Many Dutch people say 'enigste' when they should be saying 'enige'. This changes the meaning of the sentence a lot:
- Hij is de enige jongen = He is the only boy
- Hij is de enigste jongen = Hij is de meeste enige jongen (He is the cutest boy)
As in many other words in Dutch (especially adjectives), you have to add this extra "-e" in the end of the word according to the word it refers to. The basic rule is: If it's a de-word, you add the extra -e. If it's a het-word, you don't add.
There are other rules and exceptions, though -- In the "Adjectives Basics" lesson, there's a complete guide about it: https://www.duolingo.com/skill/dn/Adjective-Basics. I suppose the same rules apply to the pronouns we're studying in this lesson as well =) (please, someone correct me if I'm wrong)
Because "Does he speak only English" is a question if English is the only language he speaks, whereas the meaning of Dutch sentence is "if he speaks English at all".
I'm hoping a moderator will answer this question - do enig, elk and enkel all have the same rule? That is to say, 'enig', 'elk' and 'enkel' for 'het' words, 'enige', 'elke' and 'enklele' for 'de' words? Obviously these three words have their own individual uses, but is this the basis for including them on the same unit?
That is because weinig and veel can take on one of three roles in a sentence, none of which are "adjective" - determiner, pronoun, adverb. Hence why they are introduced in the indefinite pronoun unit and not the adjective unit. Determiners inflect in a different way than adjectives (in this case, they get the "-e" ending in the definite usage, ex. weinig jongens - few boys, de weinige jongens - the few boys)
Additionally, enig (among others) can be either a determiner (as it is used in this sentence) or an adjective (as it is used in some of the examples above in this discussion). The inflection for enig as a determiner is with the "-e" ending in all usages except when the noun is neuter. When used as an adjective, the normal rules apply.
It is unfortunate that a lot of the discussions on this site lead people to confusion by citing the "adjective rules" for words that do not function as adjectives in the sentence in question. :-/
Edit: One other point - enig only means "only"/"single" when it is used as an adjective. Therefore this sentence, in which enig is a determiner, can never be translated as "Does he speak only English?" but only "Does he speak any/some English?"
It has nothing to do with being close to the border or not. The word has been used in the Netherlands (the entire country) for quite some time now (at least more than 100 years) and if you just classify überhaupt as "German" here you can also get rid of quite a few words we use that originate from French and other languages..
Since you said you lived in the Netherlands I'm sure you are familiar with the following sources:
- NOS: https://nos.nl/nieuwsuur/artikel/2086913-oud-ministers-van-integratie-beleid-ontbreekt.html
- NRC: https://www.nrc.nl/nieuws/2014/04/09/interesseert-het-ons-uberhaupt-1364830-a1320324
- Volkskrant: https://www.volkskrant.nl/binnenland/kan-de-coalitie-vvd-d66-cda-en-christenunie-uberhaupt-wel-tegenspoed-verdragen~a4511341/
- Financieel Dagblad: https://fd.nl/ondernemen/1193931/ageas-vecht-om-fortis-boek-te-kunnen-sluiten
- AD: http://www.ad.nl/tv-en-radio/gevecht-mcgregor-mayweather-tenoacute-ch-te-zien-in-nederland~a6090a42/