de and el navigate.yaml still use string-length($Prefix), which breaks as soon as their prefixes are translated

[michaldziwisz]

ec36e057 replaced string-length($Prefix)+1 with $CommandOffset in
navigate.yaml, because $NavCommand is always English (ZoomIn, MoveNext)
while $Prefix is the spoken, translated word. de and el still carry the old
formula:

lang   string-length($Prefix)+1   $CommandOffset   Prefix words
de                            9                0   zoom, move, read, describe
el                            9                0   zoom, move, read, describe
en fr hu nb sv pl             0                9   (translated)

Right now both work, but only by coincidence: their Prefix values are still the
untranslated English words, so string-length("move") happens to equal the length
of the Move stem. The first translator who localizes them — the obvious thing to
do with a user-facing string — silently breaks every navigation announcement in
that language.

This is not hypothetical; it is exactly what pl was suffering from before #738.
With Prefix: 'przejdź' (7 chars) against the Move stem (4):

ZoomIn        -> ""          (want "In")
ZoomOutAll    -> "ll"        (want "OutAll")
MoveNext      -> "t"         (want "Next")
DescribeNext  -> "ibeNext"   (want "Next")

All 16 branches were dead, so users heard "przejdź; do mianownika" with no
direction word at all. Nothing failed: there were no navigation tests for pl,
and the 600+ speech tests were green throughout.

Two things might be worth doing:

1. Port $CommandOffset to de and el now, while their prefixes are still
   English and the change is a no-op that can be verified as such.
2. Since the same trap can reappear in any new language, it might be worth
   comparing against $NavCommand directly, the way ru does
   ($NavCommand = 'ZoomIn'). More verbose, but it cannot be got wrong by
   choosing a different word length.

I am happy to send a PR for point 1 if you would like it — it is mechanical, and
I would keep it to those two files.

For reference, es, fi, id, vi and zz do not use this construct at all.