@gp54321 thanks! I haven't tried it yet, and it works..but as you said i haven't found any plone/tinymce class that sets a text-align.
We could use bootstrap's classes (text-right, text-left) but seems that they are not included in barceloneta.
Going deeper in this feature, i've found that from 5.0 to 5.1 the html filters control panel has changed, and exactly this commit introduced the "problem":
in Plone 5.0 there was a "style_whitelist" field with a set of allowed styles (also text-align), but in the new control panel we have less fields and we can only enable the whole "styles" attribute.
@MrTango, i (git) blame you for this 
Obviously i don't know all the background about this decision, but is it some kind of regression or i'm missing some other configuration somewhere?
I see two possible solutions:
- add the config in tiny control panel + some css classes in the theme (if there isn't already)
- revert attributes filter and controlpanel entry
What do you think?