add missing translation keys

[Batalex]

See #4142

This PR adds the missing "Download plot" localization key for the following languages:

• cs
• de
• es
• fr
• it
• ja
• pt-br
• sw

The others languages either have already the missing key or do not translate modebar buttons.
I am by no mean a translator (Deepl/Translate for the win). Native speakers feedbacks are welcome even if it seemed "simple".

[etpinard]

@Batalex how well do you know each of the languages you edited?

I can personally approve the fr translation you provided. The de translation seems pretty straight forward too. But I don't think we should trust Deepl/Translate. In other words, I would prefer not translating Download plot at all over adding a translation that no one approved.

[etpinard]

Pinging people that could potentially help us out:

@fhurta @JonasGutermuth @n-riesco @booleanbetrayal @Domino987 @ghtmtt @jins-tkomoda @andrefarzat

[andrefarzat]

@etpinard it has my ok for the pt-br key 😸

[Domino987]

@etpinard it has my ok for de, but it would be better to be "Graphen herunterladen". Both work though.

[ghtmtt]

@etpinard +1 for Italian too

[fhurta]

@etpinard cs translation is fine

[Batalex]

Thanks everyone for your time!

@Domino987 My german is awful, but how do you feel about the "Download plot as a png" just above? Would "Graphen als PNG herunterladen" be better as well?

[Domino987]

@Batalex yes that would be better as well.

[tkomde]

@etpinard ja translation is also fine!

[n-riesco]

@etpinard lib/locales/es.js LGTM

[etpinard]

Thanks everyone!

We're only missing a 👍 from a sw speaker

[etpinard]

Ok, we're still missing a 👍 from a sw speaker, but since the translation appears straightforward

I'm ok trusting Deepl here. Merging. Set for release in 1.50.0