I’ve tried to transcribe the beginning of the song:
|I went everywhere for you||a? w?nt ?f??w?? f? ju?|
|I even did my hair for you||a? i?v?n d?d ma? h??? f? j??|
|I bought new underwear, they’re blue||a? b??t nu? and?w??, ða? blu?|
|And I wore ’em just the other day||?n ? w?r?m d??st ð? ?ð? da?|
|Love, you know I’ll fight for you||l?v, ju? n?? a?l fa?t f? ju?|
|I left on the porch light for you||a? l?ft ?n d? p??t? la?t f? ju?|
|Whether you are sweet or cruel||w?ð? ju? a? swi?t ?? k?u?(l)|
|I’m gonna love you either way||a?m ??n? l?v ju? i?ð? we?|
|Love, oh, love, I gotta tell you how I feel about you||l?v ?? l?v a? ??t? t?l ju? ha? a? fi?l ?baut ju?|
|‘Cause I, oh, I can’t go a minute without your love||k?z a? ø? ai kan ??? ? m?n?t wiða?t ju? l?v|
Some people (at least in Germany and Denmark) have been trying to defend her pronunciation by claiming she’s singing in Cockney (by which I guess they mean Estuary English), but that’s a misunderstanding.
It’s true that many native Londoners would pronounce ‘day’ in a way close to [da?], but like all other native speakers of English, they would make it rhyme with ‘way’, which should in this accent be [wa?], not [we?].
In the same way, no matter what accent of English you’re trying to speak, you should use the same vowel/diphthong in ‘oh’ and ‘go’.
Also, no variety of English that I know of conflates the vowels in ‘light’ and ‘day’. In Scottish English, they are [??] and [e]; in RP, they are [a?] and [e?]; and in Estuary English, they might be [??] and [??]. The main thing is they’re always different.
Finally, of course there are features of a London accent that she hasn’t copied at all – for instance, her /l/s and /r/s sound very German to me.
So this is not a case of a German who sings in Cockney; it’s a case of a German schoolgirl with mediocre English who’s copied pronunciations at random from songs she’s listened to.