Use appropriate decimal separator in playback speeds

[moneytoo]

The exo_playback_speeds array contains values with hardcoded decimal separator (.).

When I have my device set to Czech language (and therefore to Czech locale), I would expect to see values like 1,5x (instead of 1.5x).

While it's probably clear to users from the context, in Czech locale 1.5x would meant 1500 times.

Other apps like YouTube and Files by Google use the localized values.

EDIT: YouTube also uses actual multiplication sign × instead of the latin letter x.

[ojw28]

We should make sure these strings are translated. Thanks for reporting!

[ojw28]

We will fix this across a few of commits:

1. Add translatable strings
2. Add translations for the strings
3. Update the UI components to use the translatable strings