There are a bunch of undefined language strings in version 2.1.0 as noted:

Front End Notifications
COM_MATUKIO_BOOKING_ANNULATION_SUCESSFULL
COM_MATUKIO_SAVE

Configuration->Basic Tab
stornoconfirmation (has no hover-text, it should because I have no idea what storno is)

Configuration->Layout Tab
COM_MATUKIO__DESC
COM_MATUKIO_SAEULE
COM_MATUKIO_OLDBOOKINGFORM_DESC

Configuration->Payment
Every hover-text on the Payment tab, 17 in all.

Configuration->Advanced tab
COM_MATUKIO_CAPTCHA_DESC
COM_MATUKIO_CSV_EXPORT_CHARSET_DESC

Bookingfields
COM_MATUKIO_FIELDS_CITY
COM_MATUKIO_FIELDS_COMMENTS
COM_MATUKIO_FIELDS_COMPANY
COM_MATUKIO_FIELDS_COUNTRY
COM_MATUKIO_FIELDS_EMAIL
COM_MATUKIO_FIELDS_FAX
COM_MATUKIO_FIELDS_FIRST_NAME
COM_MATUKIO_FIELDS_SURNAME
COM_MATUKIO_FIELDS_MOBILE
COM_MATUKIO_FIELDS_PHONE
COM_MATUKIO_FIELDS_STREET
COM_MATUKIO_FIELDS_TITLE
COM_MATUKIO_FIELD_CHOOSE
COM_MATUKIO_FIELD_MR
COM_MATUKIO_FIELD_MS
COM_MATUKIO_FIELDS_ZIP

Edit / Create Bookingfield
COM_MATUKIO_TYPE_SPACER

Options
COM_MATUKIO_CONFIG_LIVE_UPDATE_HEADER_DESC
COM_MATUKIO_PERMISSIONS_FRONTEND_LABEL
COM_MATUKIO_PERMISSIONS_FRONTEND_DESC

===Other Things===
-Back End-

Password field in the Options->Update Settings is plain text.

Categories needs component nav bar:

Control Center | Events | Templates | Coupons | Configuration | Bookingfields | Statistics | Categories | Live update

On the My Bookings page, am and pm show up twice on each line:

Begin: Saturday, 18. August 2012 [color=#ff0000]am[/color] 09:00 [color=#ff0000]am[/color]
End: Tuesday, 18. September 2012 [color=#ff0000]pm[/color] 04:00 [color=#ff0000]pm[/color]
Date of booking: Tuesday, 14. August 2012 [color=#ff0000]am[/color] 10:27 [color=#ff0000]am[/color]

Fix is to edit Date format.

-Front End-

My Bookings tab
Reset button needs class="mat_button" instead of class="button".

My Offers tab
Several buttons like New Event, Duplicate and Delete need class="mat_button" instead of class="button".